docs: show example migration from bdk to bdk_wallet

[ValuedMammal]

I created a migration workflow in the form of a runnable example that does everything in one go. In the example we

1. Open an existing sqlite db
2. Get the last revealed addresses
3. Create a new bdk_wallet::Wallet with new database
4. Restore previously revealed addresses
5. Sync with electrum

fixes #1606

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[notmandatory]

Looks good, simpler and safer than dealing directly with database or intermediate text files as I'd originally proposed. A few suggestions:

1. would read better with more descriptive variable names, ie. old_wallet, new_wallet
2. should add sanity check asserts after sync for things like balances, indexes; maybe also utxos and tx ids.
3. need to update PR description with sqlite instead of sled
4. make it clearer in code comments that you're opening an "old" (will be) 0.30 wallet and a "new" 1.0 wallet

[ValuedMammal]

I tested this by first syncing a bdk wallet using electrum_backend.rs, adapted to use this tprv

tprv8ZgxMBicQKsPdy6LMhUtFHAgpocR8GC6QmwMSFpZs7h6Eziw3SpThFfczTDh5rW2krkqffa11UpX3XkeTTB2FvzZKWXqPY54Y6Rq4AQ5R8L

and persisted with sqlite. Then I ran the example in this PR, initializing BDK_DB_PATH to the old db path.

should add sanity check asserts after sync for things like balances, indexes; maybe also utxos and tx ids

I added assertions for utxos, balance, and revealed addresses. Just to note: this assumes the old database is up to date and no new information is presented as a result of syncing the new wallet.

I'm wondering whether example-crates/ is the best place for this compared to maybe a page in Book of BDK - I guess either is fine

[notmandatory]

Yes I agree it might make a better book section with companion code to take snippets from.

[notmandatory]

Maintenance version v0.30.0 of bdk crate with upgrade to rusqlite 0.31 is now published:

https://crates.io/crates/bdk/0.30.0