FEATURE REQUEST: Crypto-request for descriptors

Looking forward at the whole cycle of what happens between a transaction coordinator and 3 signers, there is one last phase where the transaction coordinator, having all the xpubs, ought to have the option send the final descriptor to all the signers (if they want it).

Not sure if this should be a 1.5 thing or something we'll learn when doing the transaction coordinator. Some signers have no state (Passport) and will not want it. Others, like Gordian Seed Tool, may need it so that it can create a backup accounts associated with seeds, or initiate a policy where it won't sign any PSBTs for accounts that it didn't get registered.

I see the possible 1.5 feature just simply allowing basic crypto-request export and import of a descriptor, then puzzle out how/when it might work more fully in a flow in a subsequent releases after we learn from the transaction coordinator.

Definitely a stretch goal.

Start by reading the transaction-coordinator flow and the existing crypto-request handling for descriptors, although the issue does not name specific files or tests. Define whether the initial scope is only basic descriptor export and import, and document how optional delivery to signers should work; the issue's broader signer-registration flow is explicitly deferred.

Written by the indexing model from the issue text.