[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