This change avoids blocking requests when retrieving covers from
SteamGridDB, which is the main bottleneck as the current implementation
iterates over paginated results for multiple games.
Using an asynchronous client like `httpx` provides a good performance
improvement, and reduces the latency when calling this endpoint.
Also, the inclusion of FastAPI `lifespan` allows instantiating a single
client on startup.
When testing with "Final Fantasy V Advance", the endpoint goes from ~9s
to ~1.5s to retrieve all covers.
The SteamGridDB API has a bug, where the `total` field for the paginated
endpoint `/v2/grids/game/:gameId` sometimes double/triple counts results
(e.g. returning `total=172` when there are actually 86 results).
To avoid this issue, this change relies on each page's number of
results. If the received page has less than the requested amount of
grids, we know that is the last page.
