Add type hinting for plugins

[cnunciato]

One thing you get used to pretty quickly when you're writing pipelines in code is the value of type hinting. As of now, however, our typing support doesn't yet cover plugins, because the SDK lacks an awareness of the full plugin ecosystem. It'd be great, for example, if you could do something like this:

Given how many plugins there are out there, and the need to be able to cover all of them somehow, it's unclear what an ideal implementation would look like, but it's probably something to start experimenting with.

[wooly]

Very half baked idea, but could such things live in a separate repo like the DefinitelyTyped repository? https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master

[cnunciato]

@wooly Seems reasonable! I think that's what @zchase had in mind also.