What this PR does / why we need it:

Adds Set as a feature type to store collections of unique values, eliminating the need to manually deduplicate lists or convert back from maps.

Changes

• Protobuf: Added 8 SET type variants (INT32_SET, INT64_SET, STRING_SET, BYTES_SET, BOOL_SET, FLOAT_SET, DOUBLE_SET, UNIX_TIMESTAMP_SET) with corresponding message types. Compiled with protobuf version 4.25.0 to maintain version consistency with the existing codebase.
• Python SDK:
  • New Set class in types.py following the same pattern as Array
  • Bidirectional type conversions in type_map.py with automatic deduplication
  • Added SetType union and enum values in value_type.py
• Tests: Unit tests for type creation, round-trip conversions, and duplicate removal
• Docs: Updated type-system.md with set types table and usage examples

Usage

from feast.types import Set, String, Int32
from feast import Field

# Sets automatically deduplicate
Field(name="visited_pages", dtype=Set(String))
Field(name="unique_categories", dtype=Set(Int32))

# Input: {1, 2, 3, 2, 1} → Stored: {1, 2, 3}

All primitive types except Map support set variants.

Misc

Protobuf changes are backward compatible. Existing LIST and MAP types are unaffected. Generated protobuf files maintain version 4.25.0 compatibility without introducing breaking changes from newer protobuf versions.

• Fixes Add set as feature type #5886

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