Adds partial sync mode to the snap syncer that filters which contracts
have their storage and bytecode synced based on the configured filter.
Key changes:
- Syncer accepts optional ContractFilter for partial mode
- Skip markers (SnapSkipped prefix) track intentionally skipped accounts
- processAccountResponse checks filter before requesting storage/code
- Healing phase uses NewPartialStateSync to respect skip markers
- Helper functions for skip marker persistence (mark/check/delete)
When partial sync is active, only tracked contracts have their storage
synced, reducing sync size from ~1TB+ to ~30-40GB while maintaining
a complete account trie for balance queries.
Part of partial statefulness Phase 2.
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