[core] Reject Variant type for primary key and partition key

[ArnavBalyan]

Purpose

• Table creation with variant primary key passes validation today
• Variant has no comparator case in codegen, keys built with it fail at runtime. Ensure we can detect it similar to Map and array at table creation and fail the creation with variant key for safety.
• Add VariantType to blacklist of logical types, so variant keys are rejected at create time.

Tests

• UT