Add connection pool to Mint HTTP adapter (turn it into a GenServer)

[danschultzer]

So the current implementation of PowAssent.HTTPAdapter.Mint doesn't pool HTTP/2 connections. For it to do that, I'll need to set it up as a GenServer. This way all connections can be kept open and all requests can be faster and more efficient. As new requests are coming in, the connections will be created if they don't exist in the GenServer state.

I haven't decided yet if the GenServer should be an extra layer on top of the current implementation so they work in parallel, or it should just replace it entirely and you are required to start it up to use Mint. I feel the latter is the better choice, since there is not really much of a point in using Mint without connection pool.

Any feedback/thoughts are welcome, I only have a superficial understanding of HTTP/2 🙂