Tezt: Add remote signer option to agnostic baker tezt test

What

This MR is one of the many necessary steps towards consolidating the octez-agnostic-baker. It introduces a new manual tezt test which simply replaces the default signing mechanism of the baker with a remote HTTP signer.

Why

How

Manually testing the MR

Run the manual tests with dune exec tezt/manual_tests/main.exe -- --file agnostic_baking.ml . Don't forget to 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