getMainnetSdk should allow provider or signer, not just signer

[agcty]

When calling getMainnetSdk it should be allowed to pass any ethers provider or signer. In our codebase we're using a custom useDapp-like library in conjunction with web3-react.

When no signer (e.g as injected by MetaMask) is available, it defaults to a StaticJsonRPCProvider.

This is very useful in combination with useSWR (or react-query) and suspense as it provides a great DX and UX:

It will actually show the data that only needs a provider and displays a fallback state if it can't fetch the data that needs a signer but will automatically show it when the user connects a wallet that injects a signer.

[agcty]

this is important for using the hook as described in #65 😎

[krzkaczor]

This is probably a reasonable improvement. The only problem is that when a user passes a provider then the whole SDK is "read-only" and calling any method that would require a transaction will result in a runtime error.

@agcty feel free to create a PR for this.