Add plugin APIs for implementations of custom event rules.

changelog.d/5440.feature

docs/sample_config.yaml

synapse/config/homeserver.py

synapse/config/third_party_event_rules.py

synapse/events/third_party_rules.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/server.py

tests/rest/client/third_party_rules.py