[SPARK-49934][SQL] Add implicit cast for accessing collated map with literal

[stefankandic]

What changes were proposed in this pull request?

Add a new case in collation type casts for retrieving values from a map where the keys are collated strings. This will allow the literal key to be implicitly cast to the appropriate collation type used in the map.

Why are the changes needed?

So explicit collate is no longer needed when accessing map elements.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

New unit test.

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

No.