Tezt: Replace Baker with Agnostic_baker module

What

Part of #7761 (closed).

In the following tests the Baker module has been replaced by Agnostic_baker, since no test here involves having two baker binaries for migration (unlike protocol_migration.ml, for instance).

For the migration cases, see the next MR: !16878 (merged).

Why

To consolidate the agnostic baker project with extra testing, and ultimately to get rid of the Baker module, since at some point we will get rid of the protocol-dependent baking binaries.

How

Manually testing the MR

CI green (or more specifically)

dune exec tezt/tests/main.exe -- --file baker_operations_cli_options.ml

dune exec tezt/tests/main.exe -- --file baker_test.ml

dune exec tezt/tests/main.exe -- --file http_cache_headers.ml

dune exec tezt/tests/main.exe -- --file nonce_seed_revelation.ml

dune exec tezt/tests/main.exe -- --file signer_test.ml

dune exec tezt/tests/main.exe -- --file synchronisation_heuristic.ml

dune exec tezt/tests/main.exe -- --file dal.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