Expose features from the script `check-live-features` to user's code

While implementing modules for `live-agent` I noticed it would be very useful to have mechanisms for validating the features required for the module, similar to `check-live-features` behaviour. 

Most of the features can already be used (`live-agent`'s `validate-settings` is an example of such), but there are some gaps, with the required live plugins validation being the most notable