Is your feature request related to a problem? Please describe.
We have an external application that makes use of a dedicated relay (noStream), and in order to facilitate the login process, we need a mechanism by which we can find out if the user is an active (paid) user on the relay

Describe the solution you'd like
A GET request on URL such as /invoices/check/:pubkey against which the check will be done and returned in a response of a JSON type.

Describe alternatives you've considered
The alternative I tried was sending a POST request every 20 seconds to the /invoice endpoint which breaks the relay. It fails to process successful payment after that kind of polling.

Additional Context
We're currently preparing a PR for submission.