[SPARK-54136][SQL] Extract plan merging logic from MergeScalarSubqueries to PlanMerger

### What changes were proposed in this pull request?

This PR extracts the plan merging logic from `MergeScalarSubqueries` to `PlanMerger` so as to other rules can reuse it.

While the plan merging logic is extracted without modification to `PlanMerger`, `MergeScalarSubqueries` required a significant adjustment. This is because [SPARK-40618](https://issues.apache.org/jira/browse/SPARK-40618) / #38093 added subquery reference tracking so as to avoid trying to merge a subquery to any of its nested subqueries. This kind of reference trancking doesn't work well with a general `PlanMerger` so this PR modifies `MergeScalarSubqueries` to use a separate `PlanMerger`s by each subquery level.

### Why are the changes needed?

To be able to reuse plan merging logic.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing UTs.

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

Yes, Claude gave me suggestions to improve documentation.

Closes #52835 from peter-toth/SPARK-54136-extract-plan-merging-logic.

Authored-by: Peter Toth <[email protected]>
Signed-off-by: Peter Toth <[email protected]>

Author: peter-toth