Allow plugins to expose their API (optionally over RPC as well)

[Zapotek]

There's no reason why plugins shouldn't be able to be controlled programmatically via their API.

After all, plugins are there to extend the framework's functionality; no sense treating them as dumb fire-and-forget components and not allowing them the option to expose their API, even over RPC if they so choose.

[Zapotek]

Pushing this to v0.5 since plugins will have to be updated to work asynchronously.

[Zapotek]

Reviewing this again, plugins don't actually need to be rewritten to work asynchronously in order for this to work. The existing threaded model of all plugins running in parallel would still work fine and async callbacks can be added for newer plugins if need be.

In addition, the Dispatcher's RPCD Handler code (that exposes RPC APIs for modular components dynamically) can be leveraged for this purpose too.

This needs much less work than originally estimated, yay.

[Zapotek]

Removing from the v0.5 milestone, it's a nice feature but not necessary, at least not yet.