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.