Implement Pinning Service API?

Instead of "just" API for uploading (e.g. via CAR-file) - I thought we might as well add pinning possibility. (PoC already implemented) This allows to pin someone else's data on "your" server.

If we do this, we might as well be a good citizen and adhere to the Pinning Service API spec (which I did in the PoC),
but how much of that do we want? (I only added PUT /pins)
More (like listing existing) might require this service to be stateful