Allow toggling source availability

[jose-elias-alvarez]

Issues

• I have checked existing issues and there are no existing ones with the same request.

Feature description

Mentioned by @David-Else in #332. We can reuse the query mechanism from the deregister API to enable / disable / toggle source availability.

Help

Yes

Implementation help

Doing this at the source level is simple. I imagine users would want diagnostics to be hidden when a source is disabled (which is as good a reason as any to move to the new diagnostics API) and regenerated when a source is enabled, for which we can maybe reuse this logic.

[jose-elias-alvarez]

f1e69d8 adds a basic version of this. I didn't implement regenerating diagnostics, which would most likely require a synthetic notification, but I'd welcome a PR for it if anyone is interested.