Implement account status endpoints (MSC3720) (#12001)

changelog.d/12001.feature

synapse/config/experimental.py

synapse/federation/federation_client.py

synapse/federation/transport/client.py

synapse/federation/transport/server/__init__.py

synapse/federation/transport/server/federation.py

synapse/handlers/account.py

synapse/rest/client/account.py

synapse/rest/client/capabilities.py

synapse/server.py

tests/rest/client/test_account.py