[SPARK-53718][SQL] Add a new Optimizer rule to flatten nested MapConcat expressions

[MRKKmrkk]

What changes were proposed in this pull request?

Add a new Optimizer rule to flatten nested MapConcat expressions in two ways:

1.Flattens nested MapConcat expressions into a single MapConcat.
Example: map_concat(map('a', 'a1'), map_concat(map('b', 'b1'), map('c', 'c1'))) → map_concat(map('a', 'a1'), map('b', 'b1'), map('c', 'c1'))

2.Extracts the child directly when MapConcat has only one child
Example: map_concat(map('a', 'a1')) → map('a', 'a1')

Does this PR introduce any user-facing change?

No.

How was this patch tested?

SQLQueryTestSuite and CombineMapConcatsSuite.

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

No.

[github-actions]

We're closing this PR because it hasn't been updated in a while. This isn't a judgement on the merit of the PR in any way. It's just a way of keeping the PR queue manageable.
If you'd like to revive this PR, please reopen it and ask a committer to remove the Stale tag!