Implement per-room message retention policies

changelog.d/5815.feature

docs/sample_config.yaml

synapse/api/constants.py

synapse/config/server.py

synapse/events/validator.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/pagination.py

synapse/storage/data_stores/main/events.py

synapse/storage/data_stores/main/room.py

...orage/data_stores/main/schema/delta/56/room_retention.sql

synapse/visibility.py

tests/rest/client/test_retention.py