Convert more cached return values to immutable types (#16356)

changelog.d/16356.misc

synapse/api/filtering.py

synapse/federation/federation_client.py

synapse/handlers/federation_event.py

synapse/handlers/relations.py

synapse/rest/client/filter.py

synapse/storage/controllers/state.py

synapse/storage/databases/main/filtering.py

synapse/storage/databases/main/relations.py

synapse/storage/databases/main/roommember.py

tests/util/caches/test_descriptors.py