[standards] Collect eCash-specific variations to BIP21 in a spec

Summary:
eCash has used BIP21 URI parameters with some obvious-ish but unspec'ed modifications (for example, using ecash address format, using units of XEC instead of BTC).

Codify these modifications. Add a new opreturn param.

Test Plan: Read and suggest improvements. Sanity check for opreturn spec, to be implemented in Cashtab.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D14905