Small refactoring of mina_block.ml and block_sink.ml

[georgeee]

Step toward #16582

Explain your changes:

• Replace Mina_block.to_yojson with Mina_block.to_logging_yojson
  • the new functions operates on headers, but pretends to dump the block-like json
• Small refactoring of block_sink.ml (just syntax)

Explain how you tested your changes:

• it compiles
• no functionality is changed

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? None

[georgeee]

!ci-build-me

[georgeee]

!ci-build-me