* Server key API (works for monolith but not for polylith yet)
* Re-enable caching on server key API component
* Groundwork for HTTP APIs for server key API
* Hopefully implement HTTP for server key API
* Simplify public key request marshalling from map keys
* Update gomatrixserverlib
* go mod tidy
* Common -> internal
* remove keyring.go
* Update Docker Hub for server key API
* YAML is funny about indentation
* Wire in new server key API into hybrid monolith mode
* Create maps
* Route server key API endpoints on internal API mux
* Fix server key API URLs
* Add fetcher behaviour into server key API implementation
* Return error if we failed to fetch some keys
* Return results anyway
* Move things about a bit
* Remove unused code
* Fix comments, don't use federation sender URL in polylith mode
* Add server_key_api to sample config
* Review comments
* HTTP API to cache keys that have been requested
* Overwrite server_key_api listen in monolith hybrid mode
