Use flask instead of aiohttp as RPC server.
The azure backend cannot be run in the same thread as an asyncio event loop. Resolves #3981 (closed)
Additionally, this simplifies the code, by reusing the same method auto-generation as other SWH components.
Depends on swh-core!249 (closed), !93 (closed), !94 (closed), swh-core!250 (closed)
Test Plan
will fail because it depends on two diffs in swh-core
Migrated from D7272 (view on Phabricator)