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.

[junderw]

I'm going to merge this as-is.

The dummy signature thing is a bit of a sore spot, but since we don't officially support this and it's just a simple example, anyone who wants to do this can look at the example and do it themselves.