Technically, this allows to drop redundant code between rpc server and
rpc client using the base class from swh.core.api.asynchronous.RPCServerApp (which
now hides the detail of aiohttp there). This now matches how other rpc servers
are declared, reducing friction on reading.
Ultimately, the goal is to make this maintenance a bit less heavy than it currently is.
This also adds types so it aligns with other modules.
Related to T2223