Add a `renku plugin add` command

[Panaetius]

We should have a renku plugin add <name> command that adds a plugin to the project metadata as being part of a project.
We should check that the plugin actually exists on pypi.

I'm not sure how we'd deal with versioning (how to pin it, version ranges etc.).
Also needs a renku ontology definition for what the field on project metadata for storing this will be called and what shape it takes.

Think requirements.txt but in project metadata (next to template_source etc) and specifically for plugins.