[Feature request] Check if userid is subscribed

[josemi-ca]

It would be very useful if the library included a simple function check if there's already a subscription with a certain user id.

This would make it possible to check server-side if it is necessary to prompt the user to enable them.

I know it's already possible to check using JS or the REST API but it would be handy for certain scenarios if a simple function for this was included in the PHP library.