Add API to subscribe settled invoices

[yzernik]

Something like the equivalent of SubscribeInvoices in LND: https://api.lightning.community/#subscribeinvoices

This will probably require gRPC, because it has a streaming response.

[johncantrell97]

yeah we definitely need to add support for some grpc stream apis but for each one we add it needs to also have a websocket equivalent.

[johncantrell97]

similar to how there is the generic NodeRequest/NodeResponse and AdminRequest/AdminResponse enums we will probably want generic mpsc::Channel's for sending/receiving this data over.

I think there probably needs to be an issue about setting up streaming apis in general that needs to happen in order to properly implement this and other endpoints.

[yzernik]

An alternative way to implement this is to do what c-lightning does:
https://lightning.readthedocs.io/lightning-waitanyinvoice.7.html

The waitanyinvoice RPC command waits until an invoice is paid, then returns a single entry ...