Support for MSC2285 (hidden read receipts) (#10413)

changelog.d/10413.feature

synapse/api/constants.py

synapse/config/experimental.py

synapse/handlers/initial_sync.py

synapse/handlers/receipts.py

synapse/replication/tcp/client.py

synapse/rest/client/v2_alpha/read_marker.py

synapse/rest/client/v2_alpha/receipts.py

synapse/rest/client/versions.py

tests/handlers/test_receipts.py

tests/rest/client/v2_alpha/test_sync.py