[Cashtab + doc] Parse XECX rewards

Summary:
Add spec for XECX EMPP txs

Implement spec in Cashtab to parse XECX payout txs

To do this, we implement parsing EMPP app actions in Cashtab with associated helper functions.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17407