Add an MQTT server (aka 'broker') that logs all published messages. This
is purely for debugging Flamenco Manager's events. Although it should
work fine as MQTT broker, it's not meant for production use.
Run the server with `go run ./cmd/mqtt-server`
Add a PUT method for `/api/v3/configuration/file`, which entirely
replaces `flamenco-manager.yaml` with the received JSON payload. This
will be used in the future to store configuration edited in the web
frontend.
Ref: #99426
Reviewed-on: https://projects.blender.org/studio/flamenco/pulls/104406
Set the default MQTT topic prefix to 'flamenco'. It can still be overridden
by the config in the YAML file, but it's nice to have a sensible default
when people don't configure this.
Send events on Manager startup & shutdown. To make this possible, events
sent to MQTT are now queued up until an MQTT server can be reached.
Otherwise the startup event would be sent before the MQTT connection was
established.
