feat: implement xpub network consistency check

[nseguias]

Description

This PR addresses issue #844 by implementing a network consistency check for extended public keys (xpubs) in descriptors. The change introduces an XKeyNetwork enum and a xkey_network() method on Descriptor<DescriptorPublicKey> and Descriptor<DefiniteDescriptorKey> to:

• Detect the presence of xpubs.
• Identify if they belong to a single network (e.g., mainnet or testnet).
• Flag cases where xpubs have mixed networks, which could lead to unsafe usage (e.g., sending mainnet funds to testnet addresses).

The implementation iterates over keys to determine consistency, returning XKeyNetwork::NoXKeys, XKeyNetwork::Mixed, or XKeyNetwork::Single(Network).

Changes

• Added XKeyNetwork enum to represent network states.
• Implemented xkey_network() method.

Testing

• Added unit tests to verify behavior with single-network, mixed-network, and no-xpub descriptors.
• Tested against existing descriptor examples to ensure compatibility.

Notes

• This is a safety feature and does not enforce network consistency at parse time by default.

Related Issues

• Closes Network checking on DescriptorPublicKey and DefiniteDescriptorKey #844

[apoelstra]

Nice! In general we prefer the detailed messages to be on both the PR description and the commit message. For a single-commit PR it's fine. (And you could've reduced the detail, but everything here is somewhat informative even if it's wordy and has a bunch of unnecessary headings.)

[apoelstra]

ACK 497e994; successfully ran local tests

[nseguias]

Noted! Thanks for reviewing & testing