blip-0026: L402 - Lightning HTTP 402 Protocol

In this commit, we add a bLIP for the L402 (formerly known as LSAT)
protocol. The L402 protocol presents a standardized way of adding LN
micropayments to any existing HTTP-REST or gRPC API. The L402 protocol
repurposes the HTTP 402 Payment Required error code with the necessary
authentication headers required to bind a request's validity to the
payment of an LN invoice. Macaroons are used as flexible authentication
credentials (supports custom caveats, attenuation, etc) which allow an
L402 reverse-proxy to validate an L402 API key without backend LN node
interaction for each request (the macaroon commits to the payment hash).

Author: Roasbeef