feat: use new `NetworkKind` throughout the BDK codebase

[oleonardolima]

Describe the enhancement

In the new release of rust-bitcoin 0.32.0 the new NetworkKind type has been introduced, it's not being explicitly used in the PR bitcoindevkit/bdk#1448, but it should be instead of simply using conversions.

Use case

Use the proper rust-bitcoin NetworkKind type/API instead of multiple conversions.

Additional context

[oleonardolima]

dev call: these changes are not a high priority, but could land in future releases.

[luisschwab]

@oleonardolima are you working on this? If not I can pick it up.

[oleonardolima]

I'm not actively working on this one; feel free to pick it up. However, as a heads-up, it'll probably be a breaking change and only land on 3.0.0 as 2.0.0 is due now in June.