Add package for client-side functionality

[philippgille]

Currently when a developer uses an API that's paywalled with ln-paywall (or ElementProject's paypercall and potentially other projects in the future), he has to take care of:

• Sending two physical requests for one logical one
• Handling the 402 response
• Establishing a connection to his Lightning Network node by himself and calling its RPC / HTTP API

All by himself.

This can be easily wrapped in a way so that the developer only has to create one instance of a client, similar to the net/http.Client, create a single net/http.Request and then call client.Do(...). Just as with the net/http.Client. All the payment handling can happen automatically in the background.

The first goal should be a minimal working version. More advaned features should be added in the future with separate issues created for each feature.

Minimal working version:

• Connects to and works with lnd
• No convenience methods (Get(...), Post(...)), just Do(...)
  • Are they exposed by the embedded HTTP client? Can they be hidden in that case?
• No configuration like "only pay if the amount in the invoice is <= x BTC", just always pay the invoice

Future features:

• Support other LN node implementations
• Make payments configurable, e.g. "only pay if the amount in the invoice is <= x BTC"
• ...

[philippgille]

In the future it should probably be in its own repository, but for now it's okay to keep the packages close to each other.