[SPARK-48700] [SQL] Mode expression for complex types (all collations)

[GideonPotok]

What changes were proposed in this pull request?

Add support for complex types with subfields that are collated strings, for the mode operator.

Why are the changes needed?

Full support for collations as per SPARK-48700

Does this PR introduce any user-facing change?

Yes.

How was this patch tested?

Unit tests only, so far.

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

No.