[Cashtab] Add function to generate targetOutputs for multisend tx from user input
Summary: Diff required to implement new send functions. Convert user input into desired targetOutputs for new ecash-coinselect based tx generation fn.
Test Plan: npm test
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D14976