CM-778 - Add multicaller contract with tests (#234)

* Add multicaller contract with tests

* Added threat model

* Fixing some errors with the tests

* Fixes npm error

* Add high level image

* Fix solhint warnings/errors

* Fix slither warnings

* Add Readme.md inside contracts/multicall

* Modify readme to add threat model

* Another attempt to silence slither warning

* Add exclusion for Slither Detector calls-loop

audits/multicall/202309-threat-model-multicaller.md

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+## Background
+---
+Currently the Primary Sales product uses the pattern of Multicall to transfer ERC20 tokens and mint ERC721/ERC1155 tokens in a single transaction. The Multicaller contract is a common pattern in the space and is used by many projects, this has some added security features that will be mentioned below.
+
+## Architecture
+---
+### Contract High-Level Design
+---
+![alt text](202309-threat-model-multicaller/high-level.png "High Level")
+
+The core of the `Guarded Multi-caller` system is `Multi-call`, allowing for the minting and burning of multiple NFTs from various collections in a single transaction. Due to the high level of security needed when working with NFTs, additional safety measures have been implemented to meet security standards.
+
+- `Function Permits` prevent destructive calls from being made from the Multi-caller contract.
+- `Signature Validation` prevents multi-call instructions from random parties and only allows multi-call instructions from trusted parties.
+- `Signer Access Control` manages these trusted parties.
+- `References` provide anti-replay protection and help sync with any web2 system listening to events.
+
+### System High-Level Design
+---
+![alt text](202309-threat-model-multicaller/architecture.png "Architecture")
+#### Components
+---
+| Component                    	| Ownership 	| Description                                                                                                             	|
+|------------------------------	|-----------	|-------------------------------------------------------------------------------------------------------------------------	|
+| Client                       	| Customer  	| This can be a game client or a mobile client that the players interact with.                                            	|
+| Central Authority            	| Customer  	| It generates a list of function calls to be executed and gets a valid signature for those calls from `Multi-call Signer`. 	|
+| Multi-call Signer            	| Customer 	  |  It takes a list of function calls and generates a valid signature using a `EOA` with `MULTICALL_SIGNER_ROLE`.              	|
+| Guarded Multicaller Contract 	| Customer  	| It validates an input signature and executes an authorized list of function calls.                                      	|
+
+#### Flow
+---
+Let’s look at the flow for basic crafting, where players burn one NFT from the `ERC721Card` contract and mint a new NFT on the `ERC721Pet` contract:
+
+1. An `EOA` with `DEFAULT_ADMIN_ROLE` calls the `Guarded Multi-caller` contract to permit mint function `mint(address,uint256)` with the `ERC721Pet` contract.
+2. An `EOA` with `DEFAULT_ADMIN_ROLE` calls the `ERC721Pet` contract to grant `MINTER_ROLE` to the `Guarded Multi-caller` contract.
+3. A client requests the `Central Authority` to generate a list of function calls to burn and mint and request a signature from the `Multi-call Signer`.
+4. The `Multi-call Signer` uses an account with `MULTICALL_SIGNER_ROLE` to sign the function calls and returns the signature back to the `Central Authority`. 
+5. The `Central Authority` returns the list of function calls and the signature to the client.
+6. The `Client` approves the `Guarded Multi-caller` contract as a spender for their `ERC721Card` NFT.
+7. The `Client` submits a transaction to the `Guarded Multi-caller` contract to execute the list of function calls.
+8. The `GuardedMulticaller` contract calls the `ERC721Card` contract to burn the NFT and calls the `ERC721Pet` contract to mint a new NFT to the player’s wallet.
+
+## Public Interface
+---
+```javascript
+function execute(
+    address _multicallSigner,
+    bytes32 _ref,
+    address[] calldata _targets,
+    bytes[] calldata _data,
+    uint256 _deadline,
+    bytes calldata _signature
+)
+
+function setFunctionPermits(
+    address[] calldata _targets,
+    bytes4[] calldata _functionSelectors,
+    bool[] calldata _permitted
+)
+
+function isFunctionPermitted(
+    address _target,
+    bytes4 _functionSelector
+)
+
+function grantMulticallSignerRole(
+    address _user
+)
+
+function revokeMulticallSignerRole(
+    address _user
+)
+```
+
+## Security Considerations
+---
+- If the `Central Authority` is compromised, attackers are able to generate valid signatures to mint new tokens for themselves. As each customer manages their own `Central Authority`, its impact is per customer.
+- If the `Multi-call Signer` is compromised, attackers are able to generate valid signatures to mint new tokens for themselves. As each customer manages their own `Multi-call Signer`, its impact is per customer.

contracts/mocks/MockFunctions.sol

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+// SPDX-License-Identifier: Unlicense
+// This file is part of the test code for GuardedMulticaller
+pragma solidity ^0.8.19;
+
+contract MockFunctions {
+    // solhint-disable-next-line no-empty-blocks
+    function succeed() public pure {
+        // This function is intentionally left empty to simulate a successful call
+    }
+
+    function revertWithNoReason() public pure {
+        // solhint-disable-next-line custom-errors,reason-string
+        revert();
+    }
+
+    // solhint-disable-next-line no-empty-blocks
+    function nonPermitted() public pure {
+        // This function is intentionally left empty to simulate a non-permitted action
+    }
+}