Add taproot multisig with verified unspendable internalPubkey example

[junderw]

@motorina0 @jasonandjay Please review when you have time.

@motorina0 I wonder if we can modify PSBT in a way that automates the dummy signature process during finalization?

At the very least maybe we can make it easier to create and insert the dummies... this feels pretty awkward.

Also, "Nothing Up My Sleeve" internal key is not easy to understand or come across if you only read our examples and code, maybe this can be baked into a new payment someday... like p2trms or something.