Add a new service adapter for the IGDB API, to separate concerns with
RomM's handler for metadata. This adapter is agnostic to the handler and
only provides methods to interact with the API, and correctly return
typed responses.
The API authorization was also improved to not rely on decorating each
method that makes requests, but instead using an `aiohttp` middleware
to automatically add the required headers to each request.
Utils `mark_expanded` and `mark_list_expanded` where added to help
narrow the types of IGDB's expandable fields when we know they are
expanded, for `mypy` type checking.
