Ratelimiting of remote media downloads (#17256)

changelog.d/17256.feature

docs/usage/configuration/config_documentation.md

synapse/config/ratelimiting.py

synapse/federation/federation_client.py

synapse/federation/transport/client.py

synapse/http/matrixfederationclient.py

synapse/media/media_repository.py

synapse/media/thumbnailer.py

synapse/rest/client/media.py

synapse/rest/media/download_resource.py

synapse/rest/media/thumbnail_resource.py

tests/media/test_media_storage.py