Pallet to store and retrieve a list of NBV vaults

[mgravitt]

create_vault Extrinsic - takes a set or BoundedVec of accounts, a string label, and the signature threshold.

(we may also have a T::ClassId to categorize vaults similar to the Uniques pallet. 🤔 )

I think that we need to copy the xpub from the profile at the time that vault is created. If the user later changes their xpub, this particular vault should remain AS IS.