Add Support for BTC Multisig

[shekohex]

Currently, Airgap does not support Bitcoin Multi-Signature, which is a very common when you are holding a big amount of bitcoin and want to secure it.

We would like to see support for

• P2SH (Legacy Multisig)
• P2WSH (Native SegWit Multisig)
• P2SH-P2WSH (embedded Segwit Multisig)

Since we are using bitcoin.js which is already supports multisig like here https://github.com/bitcoinjs/bitcoinjs-lib/blob/1f92ada3fda587c1c0a6aa93649afa04e8382b93/test/integration/transactions.spec.ts#L204