Add `supports` method on nwc client to easily check if it supports a certain NIP-47 capability

[rolznz]

It will have to do a request to get_info. And we might have to consider caching so it's not called multiple times? or maybe the NWC client should always do a get_info request when initializing?

[rolznz]

Update: this is also an issue for the new LNClient class which will fetch getInfo at the start of onPaid to check if notifications are supported. See thread: #357 (comment)