[standards] Add spec for PayButton txs

Summary: PayButton is looking for a standardized way to encode certain data into OP_RETURN fields to increase app functionality.

Test Plan: Read and comment. Let me know if I am missing some intended use, or if there is some current use case that this spec would miss.

Reviewers: #bitcoin_abc, Klakurka

Reviewed By: #bitcoin_abc, Klakurka

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D15176