Asynchronous Uploads (#15503)

changelog.d/15503.feature

docs/usage/configuration/config_documentation.md

synapse/api/errors.py

synapse/config/ratelimiting.py

synapse/config/repository.py

synapse/media/_base.py

synapse/media/media_repository.py

synapse/rest/media/create_resource.py

synapse/rest/media/download_resource.py

synapse/rest/media/media_repository_resource.py

synapse/rest/media/thumbnail_resource.py

synapse/rest/media/upload_resource.py

synapse/storage/databases/main/media_repository.py

tests/media/test_media_storage.py