Implement Bitcoin testnet node for project testing

[mgravitt]

Host a bitcoin testnet node on one of our Google VMs or another server. Sync bitcoin node.

This BDK tutorial explains the steps, but note that this is for the regtest, which is different than testnet. Regtest is meant as a local testing network, where testnet is a true global network. We will want the full testnet node(s) for our integration testing and deployment.

This tutorial may be helpful as it walks through a full testnet PSBT multisig transaction: https://bitcoindevkit.org/blog/2021/02/spending-policy-demo/