Implementation of HTTP 307 response for MSC3886 POST endpoint (#14018)

changelog.d/14018.feature

synapse/config/experimental.py

synapse/config/server.py

synapse/handlers/sso.py

synapse/http/server.py

synapse/http/site.py

synapse/rest/__init__.py

synapse/rest/client/rendezvous.py

synapse/rest/client/versions.py

synapse/rest/key/v2/local_key_resource.py

synapse/rest/synapse/client/new_user_consent.py

synapse/rest/well_known.py

tests/logging/test_terse_json.py

tests/rest/client/test_rendezvous.py

tests/server.py

tests/test_server.py