Add conditional snapshot support to Vitess connector

## Feature request or enhancement

**Which use case/requirement will be addressed by the proposed feature?**

Add conditional snapshot support to the Vitess connector, allowing users to snapshot a subset of data rather than the full table.

This is useful for:
- Backfilling data gaps after a service incident without re-ingesting full history
- Snapshotting only recent data for large tables

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**Implementation ideas (optional)**

Two approaches:

1. **Start copy from a specific PK** (e.g., `id > 1000000`)
- Use VStream's `table_p_ks` in the VGTID to specify the starting point
- This would be a Vitess-specific feature

2. **Filter with WHERE clauses** (e.g., `created_at > '2024-01-01'`)
- VStream already supports this natively; just needs config exposure in the connector