[MOD-10236] Add serialization to SVS index

[lerman25]

This PR refactors the serialization logic for vector indexes by introducing a clear abstraction layer via the Serializer base class. Both HNSWSerializer and SVSSerializer now inherit from this interface, enabling:

• Unified structure for save/load operations across index types
• Encoding version support for backward/forward compatibility
• Cleaner separation of shared vs. index-specific serialization logic
• Easier extension for future index types

Each index (e.g., HNSWIndex, SVSIndex) implements its own saveIndexFields method to handle template- and implementation-specific data.

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This PR also introduces support for saving/loading SVS indexes and validating their internal consistency.
Specifically:

• Implements the Serializer interface for SVS, enabling saveIndex() and loadIndex() methods. (Using a commit created by @rfsaliev )
• Adds a checkIntegrity() method for runtime validation of index structure and metadata.

[CLAassistant]

All committers have signed the CLA.

[meiravgri]

Awesome!
main comments are regarding visibility :)
missing review for svs_extensions.h and svs_utils.h, will go over with @rfsaliev