RPC payment

The RPC payment system in monerod is a pseudo-deprecated system that allows node operators to be compensated for RPC usage.

Although this system is pseudo-deprecated, monerod still generates related fields in responses. Cuprate follows this behavior.

However, the associated endpoints and actual functionality are not supported by Cuprate. The associated endpoints will return an error upon invocation.

TODO: decide on behavior and document https://github.com/Cuprate/cuprate/pull/233#discussion_r1700870051.