Add dashboard syncing in background goroutine

[sd2k]

This adds an option of running the dashboard syncing process inside the plugin in a background goroutine.

The big disadvantage is that the process isn't started until the first time the plugin is called for any reason, e.g. a health check or a metrics call. (This might actually help if we did end up choosing to run this in cloud though, because we could have better control over when the processes started running, rather than it just being arbitrary depending when the Grafana instance restarts).

It's good enough to run once and get some data synchronised into qdrant/vectorapi, at least! To test, run mage && npm install && npm build && docker-compose up -d --build locally then:

$ # Build the plugin
$ mage
$ npm install
$ npm run build
$ # Start Grafana and qdrant
$ docker-compose up -d --build
$ # Send a request to search for dashboards. The first request will start the sync.
$ curl http://admin:admin@localhost:3000/api/plugins/grafana-llm-app/resources/vector/search -d '{"collection": "grafana.core.dashboards", "text": "Mimir usage"}'
{"results":[{"payload":{"description":null,"panels":[{"description":"CPU usage of Mimir","title":"CPU usage"}],"title":"Mimir resource usage"},"score":0.7932371497154236}]}