Add prototype implementation of CometMock

[p-offtermatt]

Closes cosmos/interchain-security#917

Description

This PR implements a prototype for CometMock, a Go implementation of Tendermock https://github.com/informalsystems/tendermock.

Its goal is to replace Tendermint in End-To-End tests.
It provides the essential RPC functions that Tendermint provides,
and itself calls an application via ABCI over grpc.

Structure of the implementation

The files are split into three parts:

• ABCI Client:
  • cometmock/abci_client/client.go: Handles calling ABCI endpoints of the application
• RPC Server:
  • cometmock/rpc_server/rpc_server.go: Mostly code taken from cometbft. Provides an RPC server. This supports a subset of the calls outlined in https://docs.cometbft.com/v0.34/rpc/#/, which will grow over time as necessary. This is mostly boilerplate. Eventually, this should also support extra and more nuanced calls, like a way to AdvanceTime,
    or to SetSigners for the next block
  • cometmock/rpc_server/routes.go: The actual implementation of the RPC endpoints. This is the less boilerplatey part of the server
• Command Line Interface
  • Logic for starting CometMock. Lives in cometmock/main.go

Implemented functionality

CometMock is essentially on feature parity with the old implementation of Tendermock.
It can:

• Interface with multiple applications
• Check their responses for nondeterminism (this is likely a bit too sensitive right now, since fields are included in the check that are allowed to be nondeterministic, but no application actually behaved nondeterministically even there in my tests yet)
• Broadcast transactions
• Perform ABCI queries
• Query for blocks

Many responses are not faithful yet, i.e. they may return results with fields that are empty even when CometBFT behaviour would be to return information in those fields. These will be included as they are necessary for tests.

[MSalopek]

This is awesome!

I hope this gets used soon, it will greatly reduce the runtimes of e2e tests and allows us to run non-determinism tests on basically every commit to figure out if there are any state-breaking change