Add bitcoind Support as a Backend Option in btcwallet

[dwertent]

Referring to #839, I’d like to propose a feature request—I noticed that a significant portion of Bitcoin functionality is already implemented in the btcwallet codebase. Would it be possible to add built-in support for bitcoind as a backend to make it work out of the box?

I'm potentially interested in contributing to this, but I'd like to hear from the maintainers first—was this omission intentional, or is there a specific reason it hasn't been implemented?

One approach I had in mind is introducing a --backend flag that accepts btcd or bitcoind, with btcd as the default. The configuration file would then be parsed accordingly based on the selected backend.

Looking forward to feedback on whether this is something worth adding to this great project!