Server key component (#1050)

* 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

build/docker/hub/docker-compose.polylith.yml

@@ -131,7 +131,7 @@ services:
       - internal
       
   key_server:
-    hostname: key_serverde
+    hostname: key_server
     image: matrixdotorg/dendrite:keyserver
     command: [
       "--config=dendrite.yaml"
@@ -141,6 +141,17 @@ services:
     networks:
         - internal
 
+  server_key_api:
+    hostname: server_key_api
+    image: matrixdotorg/dendrite:serverkeyapi
+    command: [
+      "--config=dendrite.yaml"
+    ]
+    volumes:
+      - ./config:/etc/dendrite
+    networks:
+      - internal
+
 networks:
   internal:
     attachable: true