[Apps][Examples] Sending XECs

Summary:
An example using the bitgo transaction builder to:

• collate XEC input UTXOs (filtering out eToken UTXOs)
• generate tx outputs
• sign the individual UTXOs
• build the transaction into raw hex
• broadcast the raw hex via chronik onto the network

Test Plan:
npm test
pre-fund an existing wallet and copy wallet address and mnemonic into sendXec.js
npm run sendXec <destinationAddress> <sendXecAmount>

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: bytesofman, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14198