ILP-over-HTTP Authentication

#1

I have submitted a PR to the RFCs repo that updates and clarifies the Authentication section of the RFC to more clearly define what authentication mechanisms ILP Nodes should support in order to be IL-RFC-35 compliant.

The intention is to improve the overall security of the protocol, ensure that the various Connector implementations are interoperable, and provide some security-related best-practices for Connector operators when deploying ILP-over-HTTP.

Feedback welcome, either here or in the PR: https://github.com/interledger/rfcs/pull/531