Support for testnet4 and Legacy RPC Compatibility with bitcoind Backend

[dwertent]

Overview

This PR introduces support for testnet4, as recently introduced by the Bitcoin community. Additionally, it enhances compatibility by enabling support for Legacy RPC, allowing the wallet to connect to the bitcoind backend in addition to the existing btcd backend.

Key Changes

If requested, I can split this work into two separate PRs.

• Testnet4 Support:

  • This implementation imports the testnet4 support from the PR opened against btcd (feat: Add testnet4 chain support (recreated) btcd#2318).
  • Once that PR is merged, this PR will enable full support for testnet4 in btcwallet.
  • We should update the go.mod import accordingly once the upstream PR is merged.

• Backend Selection Enhancement (Fixes Add bitcoind Support as a Backend Option in btcwallet #983 ):

  • Adds support for bitcoind as a backend, in addition to the existing btcd backend.
  • Implements the --backend flag to allow users to select their preferred backend.

How to Use

• To run on testnet4, specify the --testnet4 flag.
• To select a backend, use the --backend flag:

  --backend=btcd   # Default backend
  --backend=bitcoind # New option

Impact & Compatibility

✅ No breaking changes – existing behavior remains unchanged.
✅ Fully backward-compatible – extends functionality without modifying interfaces or APIs.

Once the btcd PR #2318 is merged, we should update the go.mod dependency to finalize testnet4 support.