Extend wallet::export module to also import/export the Caravan wallet format

[notmandatory]

Currently the wallet::export module can import and export the wallet format used by FullyNoded.

In order to support users of the Caravan multisig coordinator and to possibly migrate that project to using a WASM build of BDK we should add support for importing and exporting the Caravan wallet config json file format.

Note, the descriptor::policy module should be able to supply all of the bdk wallet descriptor information needed to create a caravan wallet config file.

[notmandatory]

@bucko13 is there a spec or docs describing the caravan config file format out there somewhere? If not it's ok, I've already managed to reverse engineer it well enough to create an equivalent descriptor but I want to make sure I'm not missing anything.

[bucko13]

No spec, no. In the true spirit of bitcoin, the json is the spec 😅

[afilini]

I've opened #589 to deprecate WalletExport and rename it to FullyNodedExport. I think we should have time to get this into the next release which will be published in a few days, so in the following one we can completely remove WalletExport to avoid causing confusion.

[bucko13]

Nice!