## What changes were proposed in this pull request?
Many Spark SQL users in my company have asked for a way to control the number of output files in Spark SQL. The users prefer not to use function repartition(n) or coalesce(n, shuffle) that require them to write and deploy Scala/Java/Python code. We propose adding the following Hive-style Coalesce and Repartition Hint to Spark SQL:
```
... SELECT /*+ COALESCE(numPartitions) */ ...
... SELECT /*+ REPARTITION(numPartitions) */ ...
```
Multiple such hints are allowed. Multiple nodes are inserted into the logical plan, and the optimizer will pick the leftmost hint.
```
INSERT INTO s SELECT /*+ REPARTITION(100), COALESCE(500), COALESCE(10) */ * FROM t
== Logical Plan ==
'InsertIntoTable 'UnresolvedRelation `s`, false, false
+- 'UnresolvedHint REPARTITION, [100]
+- 'UnresolvedHint COALESCE, [500]
+- 'UnresolvedHint COALESCE, [10]
+- 'Project [*]
+- 'UnresolvedRelation `t`
== Optimized Logical Plan ==
InsertIntoHadoopFsRelationCommand ...
+- Repartition 100, true
+- HiveTableRelation ...
```
## How was this patch tested?
All unit tests. Manual tests using explain.
Author: John Zhuge <jzhuge@apache.org>
Closes#21911 from jzhuge/SPARK-24940.
36ea55e97e