Abstract out REST proxy

[alexbosworth]

The original REST proxy mode has some problems, needed fixes:

1. Abstract out and fill in all REST translations
2. Add HMAC based authentication option
3. Add a proxy call that allows simply tunneling gRPC calls through REST

[dennisreimann]

Regarding point 3: I've experimented with that in a project of mine where I'm also using ln-service. I've written up some more on the approach here and maybe you find this interesting – relevant part here.

[alexbosworth]

Looks great

This is how ln-service started originally before I switched it to focus on gRPC. I think REST is more natural and flexible for a lot of people, including myself before I adapted my stuff to gRPC

[alexbosworth]

I've created an alternative to the REST proxy that I think can serve as a successor: https://github.com/alexbosworth/lightning/tree/master/lnd_gateway

The big difference though is that I don't really do REST with this, it's more of a direct proxy to the gRPC. That helps ensure that the REST requests would have total feature parity with gRPC requests

[dennisreimann]

Nice, I‘ll take a look!