Add custom derivation path support for QR flow

Hi,

Casa.io is interested in adding more air gapped device support to its multisig vaults, including Jade. I know that via serial Jade supports exporting with custom derivation paths, but we are interested in adding support for Jade's QR flow.

The way Casa works is:
* Casa requests an xpub at m/45'.
* Casa's backend performs derivations for each vault+subaccount.

Are you open to a firmware update for Jade's QR flow that would allow users to export an xpub at m/45'?
(could be manually entered or they could select 'Casa multisig', some wallets such as Passport do this)

If so I could start working on a pr for it following your conventions and am open to your feedback.
Btw I've contributed to Blockstream on the Core Lightning side of things https://github.com/ElementsProject/cln-application/graphs/contributors?all=1.

Start by reviewing Jade's existing QR export flow and the serial support for custom derivation paths. Determine how an xpub at m/45' could be requested, including whether manual entry or a Casa multisig option fits existing conventions. Done means the QR flow can export the requested xpub for Casa multisig vaults.

Written by the indexing model from the issue text.