Refactor server into api and service pacakges

[danielballan]

This is a long-standing idea that I think we have not yet captured in GH Issues: Separate the HTTP and FastAPI code from the rest of the Tiled service code. I like that this will:

• make the code easier to navigate
• make it possible to test the "service" separate from the HTTP API
• isolate our dependency on FastAPI, laying track for migrating to other frameworks in the far future if we need/want to

I think this will take care to do well, and I am not sure when to prioritize it, but I think it should eventually happen.