Introduce agnostic baker resilience test

This MR introduces a simple mechanism that aims to kill the baker launched by the agnostic baker during a protocol migration test and check that it is restarted automatically, such that:

• start agnostic baker
• activate-P
• bake
• kill-P-baker
• bake
• activate-P'
• bake
• kill-P'-baker
• bake
• SUCCESS

This MR is on top of !15508 (merged).

Manually testing the MR

Simply run the agnostic baker tests:

dune exec tezt/manual_tests/main.exe -- --file agnostic_baking.ml

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