Add federation option to entries and posts API routes

[jwr1]

Is your feature request related to a problem? Please describe.
Not necessarily, but I would like to be able to implement a local feed view in Interstellar, which many other Lemmy apps have. Unfortunately, it seems the routes to fetch entries and posts do not yet have the option to filter by federation status (although the magazine API actually does). Here's my related issue: interstellar-app/interstellar#130.

Describe the solution you'd like
I'd just like there to be an option to filter the items returned by these routes by federation status. The GET /api/magazines route, for instance, accepts a federation parameter with options for either all, federated, and local, with the default being all.