feat(bundles): Support backrun bundles execution

[cody-wang-cb]

Summary

• Adds a new base_sendBackrunBundle RPC endpoint and in-memory store to enable backrun transactions to execute immediately after their target transactions during block building.
• In the block building loop
  • checks the backrun bundles after executing the target transactions
  • sorts bundles in terms of priority fee
  • validates the bundle priority fee >= target priority fee
  • executes bundles in the order of priority fee
• It also should handle replacement as it replaces bundles from the same sender targeting the same tx
• Added integration tests for the backrun bundles logic

✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[dvush]

I like that this implementation is simple to understand and it solves the problem of mempool replacements in an elegant way.

There are couple of things that I wanted to point out as possible issues.

[cody-wang-cb]

not too sure if this is 100% necessary