Externalise opam admin as a plugin

[rjbou]

It is historically in opam code, but it is specific to opam repository management, not package / opam root management. It can safely be externalised as a plugin, in that case it can have its own release cycle.
When modifying opam library, we need to ensure that we don't break opam admin and that it follows opam update, and that can be done via CI jobs.

[rjbou]

First tests of extracting the opam admin to its own repo. The code is well separated, it permits to retrieve the full history.
The story/main steps

• opam-admin repo

  • extract git history of src/client/opamAdmin*.ml
  • extract git history of opam-client.opam to follow version bumps. It needs a rewrite of the opam file just to update the version with the good bump opam-client dependency
  • translate admin reftest to cram tests
  • no work needed on the build system, use directly dune, so write dune files

• opam repo

  • update doc/faq/etc.
  • remove src/client/opamAdmin*.ml code from opam repo
  • remove ancestor of opam admin command, admin-scripts/ and src/tools/admin_scripts.ml* as they are not more used, and broken (not compiling, not checked)
  • remove admin tests from reftests
  • add a ci job to check opam-admin compilation, and as opam-rt, working with a related branch for the fix

• discuss/blog post

  • Advertise in advance about the change