zcash_client_sqlite: Add a WalletMigrator that supports externally-defined migrations

[str4d]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 67.85714% with 9 lines in your changes missing coverage. Please review.

Project coverage is 54.29%. Comparing base (35c9af8) to head (6eebe35).

Files with missing lines	Patch %	Lines
zcash_client_sqlite/src/wallet/init.rs	70.37%	8 Missing ⚠️
zcash_client_sqlite/src/wallet.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1738      +/-   ##
==========================================
+ Coverage   54.27%   54.29%   +0.01%     
==========================================
  Files         179      179              
  Lines       21253    21278      +25     
==========================================
+ Hits        11535    11552      +17     
- Misses       9718     9726       +8     

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