NUMS_UNSPENDABLE_KEY_HEX is not supposed to be used as is (privacy concerns)

I believe technically the `NUMS_UNSPENDABLE_KEY_HEX` is not supposed to be used as is (privacy concerns), though for now it's fine. Per [BIP-0431](https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki#constructing-and-spending-taproot-outputs), should add an issue to do the following:

"In order to avoid leaking the information that key path spending is not possible it is recommended to pick a fresh integer r in the range 0...n-1 uniformly at random and use H + rG as internal key. It is possible to prove that this internal key does not have a known discrete logarithm with respect to G by revealing r to a verifier who can then reconstruct how the internal key was created."

_Originally posted by @notmandatory in https://github.com/bitcoindevkit/bdk-cli/pull/208#discussion_r2330503782_