[SPARK-48747][SQL] Add code point iterator to UTF8String

[uros-db]

What changes were proposed in this pull request?

Add code point iteration capabilities to the UTF8String class.

Why are the changes needed?

Need these changes to enable collation support for some expressions.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Testing new UTF8String features in UTF8StringSuite.

Was this patch authored or co-authored using generative AI tooling?

No.

[uros-db]

@allisonwang-db will do, thanks for the heads up

[uros-db]

@cloud-fan ready to merge

[cloud-fan]

thanks, merging to master!