Introduce dummy agnostic baker

This MR is a first step toward the agnostic baker, see %Protocol agnostic baker.

In this MR, we simply allow to spawn a baker accordingly to the protocol hash found in the current block. This is not yet resilient.

As the binary is experimental, it is not expected to be generated with make. Build de binary with dune build src/bin_agnostic_baker/main_agnostic_baker.exe && cp _build/default/src/bin_agnostic_baker/main_agnostic_baker.exe octez-agnostic-baker

Manually testing the MR

You can try to run the agnostic baker, with the baker arguments, in sandbox. This is not expected to be feature complete.

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