Plugin as the new entry point to plugin functionality (see details)

- plugins can now return one or more RepositoryServices, each of which
can be configured to target a different repository. this is a first
support for 'dynamic' plugins, i.e. plugins that can be configured at
deployment time or runtime to connect to multiple repositories,
typically different instances of the same category (e.g. SDMXRegistry)

- Plugin can also support Lifecycle, for cross-RepositoryService
initialisation.

- Tested the above