Add optimistic header v2 endpoint

[avalonche]

📝 Summary

Adds a header endpoint to only parse and save the header in order to construct the getHeader endpoint. A separate endpoint will be implemented as part of the optimistic v2 block submission endpoint.

Major changes:

• new submitNewHeader handler at /relay/v1/builder/headers which retains most of the checks and bid comparisons as submitNewBlock handler.
• modifies redis operations to only store the bid if the block payload is not provided (for optimistic mode)
• tests for all new functionality

TODO:

• DB operations to save the header
• submit block v2 optimistic endpoint
• demotion logic if the payload for the header was never sent for the payload

⛱ Motivation and Context

Implements the optimistic v2 proposal.

📚 References

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✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md