Add get_pre_1_wallet_keychains migration helper

[notmandatory]

Description

Add wallet/migration module and get_pre_1_wallet_keychains() to help migrate users from a pre-1.0 bdk sqlite database. This new function returns the last revealed index and checksum value for each keychain it finds.

Notes to the reviewers

This PR replaces bitcoindevkit/bdk#2090 since make more sense to put it in the wallet where it will be easier to export the bindings.

Changelog notice

• Add get_pre_1_wallet_keychains to assist migration from pre-1.0 bdk wallets.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[codecov]

Codecov Report

❌ Patch coverage is 90.27778% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.38%. Comparing base (8f8a8e9) to head (366ca7a).

Files with missing lines	Patch %	Lines
src/wallet/migration.rs	90.27%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   85.33%   85.38%   +0.04%     
==========================================
  Files          24       25       +1     
  Lines        8335     8407      +72     
==========================================
+ Hits         7113     7178      +65     
- Misses       1222     1229       +7     

Flag	Coverage Δ
rust	85.38% <90.27%> (+0.04%)	⬆️

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[thunderbiscuit]

Tested ACK 366ca7a.

[thunderbiscuit]

Part of this PR (rather a sidequest to this PR) could include updating the Migrating from 0.X page on the book of bdk, since this approach is simpler and requires less code, and I think it could simply supercede what we have there (or at least be added as a secondary section/option).

Also would be great to add a small feature page on the Release Guide when we do push the 2.4 section of the Release Guide. I keep track of the features I think should be highlighted here.

[thunderbiscuit]

Two things I'd like to clarify maybe:

1. Can we confirm the versions of bdk this will work for, and add it to the docstrings?
2. Was there a reason to return the descriptor checksum instead of the whole descriptor string?