Implement BIP70 extension: Pay for request

[schancel]

Story:
As a node operator, I want to ensure that downstream customers are not setting large numbers of keys in order to take my server offline, and bloat my DB.

Exit Criteria:
There is a standard way for a wallet to initiate a BIP70 payment for an API PUT request.

Impact:
A standardized way to pay for request will help rate limiting and spam across the internet. Potentially enabling a large number of new types of services.