[FLINK-40515][core] Extend BinaryVariant to support UUID

[manner]

What is the purpose of the change

Adds UUID (primitive type code 20) as a supported VARIANT primitive in BinaryVariant.

The 16-byte layout follows the open variant spec, which stores UUID big-endian. UUID is the only variant primitive that is not little-endian, so it uses dedicated big-endian read/write helpers.

Brief change log

• Add the UUID primitive (code 20) to BinaryVariantUtil: type mapping in getType, a case UUID in valueSize, and the getUUID reader
• Add big-endian long helpers writeLongBigEndian / readLongBigEndian; a UUID is stored as its two 64-bit halves in 16 big-endian bytes
• Add appendUUID to BinaryVariantInternalBuilder and of(UUID) to BinaryVariantBuilder
• Add getUUID() and Type.UUID to the Variant interface and of(UUID) to VariantBuilder, and wire UUID into get() and toJson()

Verifying this change

This change added tests and can be verified as follows:

• Added scalar round-trip and toJson assertions for UUID variants
• Added testUuidDecodeFromSpecBytes, which decodes the exact byte sequence from Iceberg's TestSerializedPrimitives#testUUID to confirm the wire format is compatible across implementations of the spec

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): yes (Variant and VariantBuilder are @PublicEvolving, this adds new enum constants and new interface methods)
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? not documented (documentation is tracked separately in FLINK-40494)

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CI report:

• 97e0056 Azure: SUCCESS

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[raminqaf]

@manner Thanks for adding UUID to VARIANT and competing it! Left a question! 🚀

[raminqaf]

Should we do a early check of value.length == 16 or is it always guarantied?

[manner]

Mhm, value.length should be the size of the whole variant data, not just the UUID, so I don't think we can compare to that size. And the bounds are checked inside readLongBigEndian() again if the byte[] is too short. Maybe you can clarify which check you mean.

[raminqaf]

value.length should be the size of the whole variant data

Answers my question!

[raminqaf]

Do we have any test that evaluates broken values? Like an invalid UUID?