[BTC integration] ICP canisters now have access to BTC testnet balances and UTXOs

An example of how to use this:

This is an experimental feature, currently only enabled on a single isolated subnet (2fq7c), and other subnets can use it via xnet communication (which causes most of the latency). In the future we could enable the BTC feature on multiple subnets and avoid the xnet latency.

What’s next? Right now canisters can read BTC testnet state, but not interact with it yet. Our next goal is to enable experimental threshold ECDSA signing on ICP mainnet, so canisters can sign & submit BTC testnet transactions.

note: I shamelessly copied/pasted this from Reddit post: https://www.reddit.com/r/dfinity/comments/v1tlsy/icp_canisters_now_have_access_to_btc_testnet/