[WIP] feat(network): initial gossip implementation

[dancoombs]

#248

Proposed Changes

• Add an MVP for the p2p gossip protocol. Mostly to learn about issues with the protocol specification.
• Lots of TODOs in the code that need to be addressed.

3 big spec issues currently:

1. The discovery definition doesn't explain how to find other peers that support a particular mempool. The mempool id should somehow be added to the ENR. This can either happen by somehow mapping mempool id to mempool subnet, or by adding the mempool id list directly to the ENR.
2. The definition for message_id doesn't allow the message to be sent across multiple topics. It currently gets deduplicated before being forwarded on the 2nd topic. There are 2 solutions here both with downsides.
   a. Include the topic in the message_id. This is how the latest consensus spec defines them. The downside is that the messages will get sent/received across all topics.
   b. Somehow get the libp2p spec changed to allow the same message_id to be propagated across multiple topics.

Relevant section: https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.0.md#message-cache

3. user_ops_with_entry_point is a completely invalid message type for the gossip protocol. It should be a single user operation. Else each gossiped message that has a different list will have different message IDs, each peer will then have to get each list and deduplicate the contents. Massive waste.

Going to write these things up and propose changes to the p2p spec.

[codecov]

Codecov Report

Merging #465 (9d38cc8) into danc/p2p-discv5 (6c48301) will increase coverage by 0.53%.
The diff coverage is 85.17%.

Files	Coverage Δ
crates/network/src/behaviour.rs	100.00% <ø> (ø)
crates/network/src/gossip/mod.rs	100.00% <100.00%> (ø)
crates/network/src/rpc/handler/codec.rs	94.92% <ø> (ø)
crates/network/src/rpc/handler/inbound.rs	80.43% <ø> (ø)
crates/network/src/rpc/handler/outbound.rs	89.58% <ø> (ø)
crates/network/src/rpc/handler/serde.rs	95.52% <ø> (-0.16%)	⬇️
crates/network/src/rpc/protocol.rs	84.37% <ø> (-0.25%)	⬇️
crates/network/src/gossip/message.rs	98.38% <98.38%> (ø)
crates/network/src/types.rs	97.22% <97.22%> (ø)
crates/network/src/gossip/topic.rs	94.36% <94.36%> (ø)
... and 2 more

Flag	Coverage Δ
unit-tests	56.60% <85.17%> (+0.53%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
rundler binary	0.00% <ø> (ø)
builder	45.04% <ø> (ø)
dev	0.00% <ø> (ø)
pool	59.54% <ø> (ø)
provider	1.33% <ø> (ø)
rpc	18.26% <ø> (ø)
sim	82.61% <ø> (ø)
tasks	∅ <ø> (∅)
types	93.56% <ø> (ø)
utils	7.20% <ø> (ø)
network	89.28% <85.17%> (+0.16%)	⬆️