Agnostic baker refacto/cleanup

This is a very simple refacto/cleanup MR that aims to:

• split the code into standalone modules
• add some mli files
• cleanup the code to enforce some abstractions
• fix some minor things

This MR is on top of !15305 (merged) to simplify my workflow but there is actually no real dependencies.

Manually testing the MR

Compile the uncompiled-by-default agnostic baker binary with something like rm -rf octez-agnostic-baker && dune build src/bin_agnostic_baker/main_agnostic_baker.exe && mv _build/default/src/bin_agnostic_baker/main_agnostic_baker.exe octez-agnostic-baker

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR