[SPARK-32743][SQL] Add distinct info at UnresolvedFunction toString

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

Add distinct info at `UnresolvedFunction.toString`.

### Why are the changes needed?

Make `UnresolvedFunction` info complete.

```
create table test (c1 int, c2 int);
explain extended select sum(distinct c1) from test;

-- before this pr
== Parsed Logical Plan ==
'Project [unresolvedalias('sum('c1), None)]
+- 'UnresolvedRelation [test]

-- after this pr
== Parsed Logical Plan ==
'Project [unresolvedalias('sum(distinct 'c1), None)]
+- 'UnresolvedRelation [test]
```

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

Yes, get distinct info during sql parse.

### How was this patch tested?

manual test.

Closes apache#29586 from ulysses-you/SPARK-32743.

Authored-by: ulysses <[email protected]>
Signed-off-by: Takeshi Yamamuro <[email protected]>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/unresolved.scala

@@ -268,7 +268,10 @@ case class UnresolvedFunction(
   override lazy val resolved = false
 
   override def prettyName: String = name.unquotedString
-  override def toString: String = s"'$name(${children.mkString(", ")})"
+  override def toString: String = {
+    val distinct = if (isDistinct) "distinct " else ""
+    s"'$name($distinct${children.mkString(", ")})"
+  }
 }
 
 object UnresolvedFunction {

sql/core/src/test/resources/sql-tests/inputs/explain-aqe.sql

@@ -1,3 +1,4 @@
 --IMPORT explain.sql
 
 --SET spark.sql.adaptive.enabled=true
+--SET spark.sql.maxMetadataStringLength = 500

sql/core/src/test/resources/sql-tests/inputs/explain.sql

@@ -1,5 +1,6 @@
 --SET spark.sql.codegen.wholeStage = true
 --SET spark.sql.adaptive.enabled = false
+--SET spark.sql.maxMetadataStringLength = 500
 
 -- Test tables
 CREATE table  explain_temp1 (key int, val int) USING PARQUET;
@@ -9,6 +10,11 @@ CREATE table  explain_temp4 (key int, val string) USING PARQUET;
 
 SET spark.sql.codegen.wholeStage = true;
 
+-- distinct func
+EXPLAIN EXTENDED
+  SELECT sum(distinct val)
+  FROM explain_temp1;
+
 -- single table
 EXPLAIN FORMATTED
   SELECT key, max(val) 

sql/core/src/test/resources/sql-tests/results/explain-aqe.sql.out

@@ -42,6 +42,39 @@ struct<key:string,value:string>
 spark.sql.codegen.wholeStage	true
 
 
+-- !query
+EXPLAIN EXTENDED
+  SELECT sum(distinct val)
+  FROM explain_temp1
+-- !query schema
+struct<plan:string>
+-- !query output
+== Parsed Logical Plan ==
+'Project [unresolvedalias('sum(distinct 'val), None)]
++- 'UnresolvedRelation [explain_temp1], [], false
+
+== Analyzed Logical Plan ==
+sum(DISTINCT val): bigint
+Aggregate [sum(distinct cast(val#x as bigint)) AS sum(DISTINCT val)#xL]
++- SubqueryAlias spark_catalog.default.explain_temp1
+   +- Relation[key#x,val#x] parquet
+
+== Optimized Logical Plan ==
+Aggregate [sum(distinct cast(val#x as bigint)) AS sum(DISTINCT val)#xL]
++- Project [val#x]
+   +- Relation[key#x,val#x] parquet
+
+== Physical Plan ==
+AdaptiveSparkPlan isFinalPlan=false
++- HashAggregate(keys=[], functions=[sum(distinct cast(val#x as bigint)#xL)], output=[sum(DISTINCT val)#xL])
+   +- Exchange SinglePartition, true, [id=#x]
+      +- HashAggregate(keys=[], functions=[partial_sum(distinct cast(val#x as bigint)#xL)], output=[sum#xL])
+         +- HashAggregate(keys=[cast(val#x as bigint)#xL], functions=[], output=[cast(val#x as bigint)#xL])
+            +- Exchange hashpartitioning(cast(val#x as bigint)#xL, 4), true, [id=#x]
+               +- HashAggregate(keys=[cast(val#x as bigint) AS cast(val#x as bigint)#xL], functions=[], output=[cast(val#x as bigint)#xL])
+                  +- FileScan parquet default.explain_temp1[val#x] Batched: true, DataFilters: [], Format: Parquet, Location [not included in comparison]/{warehouse_dir}/explain_temp1], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<val:int>
+
+
 -- !query
 EXPLAIN FORMATTED
   SELECT key, max(val) 

sql/core/src/test/resources/sql-tests/results/explain.sql.out

@@ -42,6 +42,39 @@ struct<key:string,value:string>
 spark.sql.codegen.wholeStage	true
 
 
+-- !query
+EXPLAIN EXTENDED
+  SELECT sum(distinct val)
+  FROM explain_temp1
+-- !query schema
+struct<plan:string>
+-- !query output
+== Parsed Logical Plan ==
+'Project [unresolvedalias('sum(distinct 'val), None)]
++- 'UnresolvedRelation [explain_temp1], [], false
+
+== Analyzed Logical Plan ==
+sum(DISTINCT val): bigint
+Aggregate [sum(distinct cast(val#x as bigint)) AS sum(DISTINCT val)#xL]
++- SubqueryAlias spark_catalog.default.explain_temp1
+   +- Relation[key#x,val#x] parquet
+
+== Optimized Logical Plan ==
+Aggregate [sum(distinct cast(val#x as bigint)) AS sum(DISTINCT val)#xL]
++- Project [val#x]
+   +- Relation[key#x,val#x] parquet
+
+== Physical Plan ==
+*HashAggregate(keys=[], functions=[sum(distinct cast(val#x as bigint)#xL)], output=[sum(DISTINCT val)#xL])
++- Exchange SinglePartition, true, [id=#x]
+   +- *HashAggregate(keys=[], functions=[partial_sum(distinct cast(val#x as bigint)#xL)], output=[sum#xL])
+      +- *HashAggregate(keys=[cast(val#x as bigint)#xL], functions=[], output=[cast(val#x as bigint)#xL])
+         +- Exchange hashpartitioning(cast(val#x as bigint)#xL, 4), true, [id=#x]
+            +- *HashAggregate(keys=[cast(val#x as bigint) AS cast(val#x as bigint)#xL], functions=[], output=[cast(val#x as bigint)#xL])
+               +- *ColumnarToRow
+                  +- FileScan parquet default.explain_temp1[val#x] Batched: true, DataFilters: [], Format: Parquet, Location [not included in comparison]/{warehouse_dir}/explain_temp1], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<val:int>
+
+
 -- !query
 EXPLAIN FORMATTED
   SELECT key, max(val)