Create wallet data migration process

[notmandatory]

Describe the enhancement

Create code and/or documentation to help pre-1.0 bdk wallet users migrate wallet data to a new 1.0 bdk_wallet. The essential information that needs to be migrated are the descriptors and their corresponding revealed indexes. Transaction and other wallet data does not need to be migrated, it can be re-loaded from a blockchain client.

The migration process should be database agnostic. Example docs should at least show how to do it with a SQLite database.

Use case

This is needed to help existing pre-1.0 bdk users upgrade to bdk_wallet 1.0+.

Additional context

This was requested by @tnull but other users will also need this feature to be able to easily upgrade their projects to the bdk_wallet 1.0+ releases.

One option is to enhance the existing wallet wallet::export code to export and import descriptors with their revealed index info.