[FEA] Add support of variant merge strategy in CAGRA

[rhdong]

Need to support various index-merging strategies (currently, only the physical merging strategy is supported).

• Logical merge via index merging: Enables merging indices without requiring a full index rebuild.
• Logical merge via query dispatching: Requires a composite_index or logically_merged_index that behaves like a CAGRA (or another supported index) but effectively distributes queries across the underlying indices.
  • In this case, the composite_index or logically_merged_index inheriting from the regular index would be recommended.