Merge different Resource implementation classes (#7732)

changelog.d/7732.bugfix

synapse/federation/transport/server.py

synapse/http/additional_resource.py

synapse/http/server.py

synapse/logging/opentracing.py

synapse/replication/http/__init__.py

synapse/replication/http/_base.py

synapse/rest/consent/consent_resource.py

synapse/rest/key/v2/remote_key_resource.py

synapse/rest/media/v1/config_resource.py

synapse/rest/media/v1/download_resource.py

synapse/rest/media/v1/preview_url_resource.py

synapse/rest/media/v1/thumbnail_resource.py

synapse/rest/media/v1/upload_resource.py

synapse/rest/oidc/callback_resource.py

synapse/rest/saml2/response_resource.py

tests/http/test_additional_resource.py

tests/test_server.py