Refactor: keyless eth rpc client

[msgmaxim]

Pull Request

Closes: PRO-889

Checklist

Please conduct a thorough self-review before opening the PR.

• I am confident that the code works.
• I have updated documentation where appropriate.

Summary

Split RpcSigningClient into EthRpcSigningClient (capable of signing/broadcasting) and RpcSigningClient (no signing). Similarly, split the traits into EthRpcApi and EthSigningRpcApi. Similary, for I split the "retry" traits into EthersRetrySigningRpcApi and EthersRetryRpcApi. Now EthersRetryRpcClient is generic over the rpc client so that if the inner client implements EthSigningRpcApi, then the retry client implements EthersRetrySigningRpcApi. The result is that EthersRetryRpcClient<EthRpcSigningClient> can do everything that EthersRetryRpcClient<EthRpcClient> plus broadcasting. Finally, I changed some of the interfaces to only require non-signing client (though a signing would work just fine) and the tracker uses a non-signing client.

I recommend reviewing this PR commit by commit.

[linear]

PRO-889 ETH Client without a signing key

In this PR: https://github.com/chainflip-io/chainflip-backend/pull/4081

we are forced to use a temp key to get around specifying one when we don't need one. We should allow initialising a client without a key, like we do for the SCC.

[codecov]

Codecov Report

Attention: 212 lines in your changes are missing coverage. Please review.

Comparison is base (aa50279) 72% compared to head (73b9215) 71%.

Files	Patch %	Lines
engine/src/eth/retry_rpc.rs	6%	103 Missing ⚠️
engine/src/eth/rpc.rs	0%	85 Missing ⚠️
engine/src/eth/rpc/address_checker.rs	0%	18 Missing ⚠️
engine/src/witness/eth/key_manager.rs	0%	3 Missing ⚠️
engine/src/witness/eth.rs	0%	1 Missing ⚠️
engine/src/witness/eth/ethereum_deposits.rs	0%	1 Missing ⚠️
engine/src/witness/start.rs	0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4256   +/-   ##
=====================================
- Coverage     72%     71%   -0%     
=====================================
  Files        385     385           
  Lines      63602   63696   +94     
  Branches   63602   63696   +94     
=====================================
- Hits       45489   45488    -1     
- Misses     15774   15863   +89     
- Partials    2339    2345    +6     

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