Author a Rust function to create output descriptor from `xpubs`

[mgravitt]

Inputs:

• set of x xpubs (or AccountIDs with xpubs saved to profile)
• threshold u8 (I believe the maximum for PSBTs is 15 signers)

Output:

• an output descriptor representing the PSBT wallet

format is:
wsh(multi( + threshold + , + XPUB_1 + , + XPUB_2 + , + XPUB_n + ))

wsh(multi(1,xpub661MyMwAqRbcFW31YEwpkMuc5THy2PSt5bDMsktWQcFF8syAmRUapSCGu8ED9W6oDMSgv6Zz8idoc4a6mr8BDzTJY47LJhkJ8UB7WEGuduB/1/0/*,xpub69H7F5d8KSRgmmdJg2KhpAK8SR3DjMwAdkxj3ZuxV27CprR9LgpeyGmXUbC6wb7ERfvrnKZjXoUmmDznezpbZb7ap6r1D3tgFxHmwMkQTPH/0/0/*))