Initial providers implementation

[lukehinds]

While #517 started the scaffolding for projects we still need to have the actual CRUD (both client and server) and actually hook this up into the server.

currently all of our provider implementations are hardcoded to GitHub (which ends up creating a GitHub client from OAuth credentials we store in the database).

The idea is to generalize the providers concept as defined in the document, and track those in the database. For now, providers should be scoped to groups, but once #542 is done we should link them to projects instead.

We'll take the provider from the object's context, and instantiate general clients when they're needed (on policy evaluation). So, we wouldn't rely on GitHub's golang implementation anymore and instead we'd have general REST and GraphQL providers that we can instantiate.

This will allow us to hook in external data sources in a more general way.

[evankanderson]

Related: #538