decouple database connection

[geospatial-jeff]

Moving the management of database resources to the client, instead of the API. Ref #57 #71

• Adds the stac_api.clients.postgres.session.Session object which uses fastapi-utils to manage database sessions and commits.
  • Previously, database sessions were managed by a FastAPI middleware and some context vars to distribute the session throughout the app. Commits were handled through PostgresClient which has been removed.
  • PaginationTokenClient is now a mixin to align with the spec (pagination is now part of core) and to re-use the database session of the parent class.
  • Note how the app factory (stac_api.api.app) no longer references any postgres related code.

[anlaksh88]

Do we need a catch-all exception handler here?

Suggested change

	raise errors.DatabaseError("unhandled database error")
	raise errors.DatabaseError("unhandled database error")
	except Exception as e:
	logger.error(e, exc_info=True)
	raise errors.DatabaseError("unhandled database error")

[geospatial-jeff]

yes good catch, we need to handle exceptions from get_db which are not of IntegrityError.

[geospatial-jeff]

catch-all exception handler doesn't work here because there are other exceptions that might get thrown within the context of a database session (like NotFoundError). These exceptions end up getting lost in the catch-all exception handler which then impacts response codes.

Instead I've updated the except to use a lower-level sqlalchemy exception. It's the lowest level that includes the wrapped psycopg2 error (e.orig) that we need to determine what error to throw. It should cover any error which occurs during the execution of a SQL statement, which I think covers our requirements here.