Add Schnorr signatures to BITBOX SDK

[cgcardona]

This Issue is tagged with a bounty of 1.1593374 BCH. Once a Pull Request (PR) has been verified to meet the acceptance criteria, and merged into the master branch, the funds will be released to the developer.

Overview

Bitcoin Cash underwent a network upgrade on May 15th. One of the new features is Schnorr signatures. This bounty is to refactor bitcoincashjs-lib and bitbox-sdk to include Schnorr signatures.

Notes

• Here's the repo for bitcoincashjs-lib
• Both TransactionBuilder and ECPair have a sign method which should get an optional signatureAlgorithms param which defaults to ECDSA.

In order for a developer to receive the bounty attached to this issue, they must meet the following acceptance criteria:

Acceptance Criteria

• A PR must be submitted to this repository with the code changes listed in the Specifications section.
• All unit tests must pass.
• All integration tests must pass.

[cgcardona]

shilch claimed this bounty w/ the below PRs.

• bitcoincashjs-lib: https://github.com/Bitcoin-com/bitcoincashjs-lib/pull/1
• BITBOX: https://github.com/Bitcoin-com/bitbox-sdk/pull/91