[Cashtab] [ECPair from wif p1/3] Add local method

Summary:
T2730

Part 1 of a 3-part stacked diff to deprecate BCH.ECPair.fromWIF function

This diff adds a local method, getECPairFromWIF, and compares this method with the existing method in the signUtxosByAddress function

Test Plan:
Review new unit test and npm test
npm start, send a transaction, look for local method matches legacy method in the dev log

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12616