[CashAddr] adds eCash prefixes to the node behind a -ecash CLI argument

Summary:
Changes node to use ecash: CashAddr prefix instead of bitcoincash: (with ectest: and ecregtest: for testnet and regressiontest, respectively) when started with the -ecash command line option

note: since the sun will set over bitcoincash: prefixes later this year, all test cases have been migrated to the ecash prefixes

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9606