[SPARK-20941][SQL] Fix SubqueryExec Reuse

### What changes were proposed in this pull request?
Before this PR, Subquery reuse does not work. Below are three issues:
- Subquery reuse does not work.
- It is sharing the same `SQLConf` (`spark.sql.exchange.reuse`) with the one for Exchange Reuse.
- No test case covers the rule Subquery reuse.

This PR is to fix the above three issues.
- Ignored the physical operator `SubqueryExec` when comparing two plans.
- Added a dedicated conf `spark.sql.subqueries.reuse` for controlling Subquery Reuse
- Added a test case for verifying the behavior

### How was this patch tested?
N/A

Author: Xiao Li <gatorsmile@gmail.com>

Closes #18169 from gatorsmile/subqueryReuse.