* Rename serverkeyapi to signingkeyserver
We use "api" for public facing stuff and "server" for internal stuff.
As the server key API is internal only, we call it 'signing key server',
which also clarifies the type of key (as opposed to TLS keys, E2E keys, etc)
* Convert docker/scripts to use signing-key-server
* Rename missed bits
* 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
