CI-in-OCaml: add Octez-specific helpers and skeleton executable in [ci/bin]

What

This MR It adds a set of Octez-specific helpers and a skeleton binary for generating the Gitlab CI YAML configuration for this repo to ci/bin. In subsequent MRs, these helpers will be used to define the CI configuration, and the binary will write the configuration when invoked.

This MR makes no change to the current CI configuration, and is just a stepping stone to the rest of the MRs in the stack.

Stack

1. !11999 (merged) (add ci/lib_gitlab_ci). Look here for rationale.
2. !12000 (merged) (add ci/bin) <- You are here
3. !10948 (merged) (translate .gitlab-ci.yml)
4. !11461 (merged) (translate latest_release pipeline)
5. !11500 (merged) (translate *release_tag(test) pipelines)
6. !11501 (merged) (translate master_branch pipeline)
7. !11534 (closed) (translate schedule_extended_test pipeline)
8. !11671 (closed) (translate before_merging pipeline)

Why

See !11999 (merged) for a discussion of Why and How.

How

By adding an executable to ci/bin. This directory contains a module for defining jobs and pipelines (in ci/bin/tezos_ci.ml(i)) in a Octez-specific manner and an empty binary, that currently does nothing.

Manually testing the MR

• dune build ci/lib_gitlab_ci
• Read the MLI files and make sure the documentation makes sense.

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