feat: persist best ChainLock to disk via MetadataStorage

[xdustinface]

Add M: MetadataStorage generic to ChainLockManager so the best validated ChainLock can be saved to disk on every update and restored on initialization.

Based on:

• refactor: cleanup storage usage in DashSpvClient constructor #418

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/chainlock-persistence

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