Run BTC sign in parallel

[Pessina]

We're facing challenges with nonce conflicts in our BTC transaction signing process due to simultaneous transactions on the NEAR blockchain. Currently, we've shifted to sequential chain signature requests to avoid these conflicts, but this workaround significantly hampers performance.

The objective of this ticket is to explore and implement a method that allows for parallel processing without nonce duplication.

Sequential signature code:

https://github.com/near/fast-auth-signer/blob/d7fb38aa8bfad5cb2528dc38641da2206e73258b/packages/near-fast-auth-signer/src/utils/multi-chain/chains/Bitcoin.ts#L431