API, Spark: Add direct UUID string-to-ByteBuffer conversion

[abhishek593]

Feature Request / Improvement

Resolve the TODO in SparkValueWriters.UUIDWriter by adding a direct UUID string-to-ByteBuffer conversion path that avoids intermediate object allocations.

Problem

The current Spark UUID write path creates two unnecessary intermediate objects per row:

1. s.toString() - allocates a String and decodes UTF-8
2. UUID.fromString() - heavy parsing (regex, splits, Long.parseLong) to create a UUID that is immediately destructured back into two longs for the ByteBuffer.

Query engine

None

Willingness to contribute

• I can contribute this improvement/feature independently
• I would be willing to contribute this improvement/feature with guidance from the Iceberg community
• I cannot contribute this improvement/feature at this time