refactor: simplify header proof generation

[morph-dev]

This is first PR that simplifies existing logic before I start adding changes from: #1779

What was wrong?

The code for generating historical_roots and historical_summaries proof is duplicated in a few places.
Some of the utility functions could also be more specific

How was it fixed?

• simplify proof generation logic in headers_with_proof and improve tests a bit
• moved the logic for generating execution_block_proof into BeaconBlock
  • removed similar, no longer needed, logic from BlockBody
• removed block generation logic from bin/e2hs-writer/src/reader.rs (it was identical with the one in headers_with_proof)
  • also removed the tests, because they were also identical
• added tests for verifying that decoded HeaderWithProof::proof matches expected value
• make build_merkle_proof_for_index generic

To-Do

• Clean up commit history and use conventional commits.

[carver]

Nice PR summary 👍🏻

Just did a light walkthrough, didn't get a deep dive. No notes!

[KolbyML]

looks good

[KolbyML]

ops wrong button before