GHA: add a job that test plugins compilation

[rjbou]

In it based on the same mechanism than opam-rt, it permits to keep them synchronised with the current API
TODO

• add a release step to check CI log for failing projects
• add a mechanism to retrieve automatically opam lib rev deps
• add a mechanism to eliminate pinned packages that rely on old versions of opam

[kit-ty-kate]

I would also suggest adding dune-release, odep, odig, odoc-driver, opam-check-npm-deps, opam-build, opam-test, opam-dune-lint, opam-graph, opam-package-upgrade, orb, opam-0install and topkg-care

[rjbou]

It was package to test (some are failing on purpose as they have a dependency on opam < 2.2, see commits message). We'll need to select what package to add here : they are blocking for the PR merge.
Ideally we would have another more light mechanism, one that sends the information as a comment in the PR for ex., like that we are aware without blocking the PR (for packages that we don't maintain).

[rjbou]

Discussion on dev meeting:
We don't want to enforce failling jobs on package that we don't maintain, but the information can be used to know which maintainer/project to ping. Ideally, it would be a job that opens an issue on the good repository, or adds a comment in the PR itself.
In the meanwhile, we would add 2 sets of dependencies to test:

• the one that we maintain as opam team or as maintainer (opam-publish, opam-build, etc.) that would hard fail if there in an API breakage
• the one that we don't maintain that would not make the job fail. It's up to us to go and look at the jobs, especially on releases
  It would also be nice to have an automatic selection of opam library rev deps from opam repo, but for that we need to add a step that eliminates package that are just incompatible (for ex ocaml-format < 2.3 soent' make sense to test).

main comment updated with TODOs