[SPARK-40980][CONNECT][TEST] Support session.sql in Connect DSL

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

This PR adds `def sql(sqlText: String)` to DSL and test the SQL proto with SparkSession/DataFrame.

### Why are the changes needed?

Improve testing coverage.

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

No

### How was this patch tested?

UT

Closes apache#38459 from amaliujia/SPARK-40980.

Authored-by: Rui Wang <[email protected]>
Signed-off-by: Ruifeng Zheng <[email protected]>

Author: amaliujia

connector/connect/src/main/scala/org/apache/spark/sql/connect/dsl/package.scala

@@ -198,6 +198,10 @@ package object dsl {
         }
         Relation.newBuilder().setRange(range).build()
       }
+
+      def sql(sqlText: String): Relation = {
+        Relation.newBuilder().setSql(SQL.newBuilder().setQuery(sqlText)).build()
+      }
     }
 
     implicit class DslLogicalPlan(val logicalPlan: Relation) {

connector/connect/src/test/scala/org/apache/spark/sql/connect/planner/SparkConnectProtoSuite.scala

@@ -230,6 +230,10 @@ class SparkConnectProtoSuite extends PlanTest with SparkConnectPlanTest {
       spark.range(2, 10, 10, 100).toDF())
   }
 
+  test("Test Session.sql") {
+    comparePlans(connect.sql("SELECT 1"), spark.sql("SELECT 1"))
+  }
+
   private def createLocalRelationProtoByQualifiedAttributes(
       attrs: Seq[proto.Expression.QualifiedAttribute]): proto.Relation = {
     val localRelationBuilder = proto.LocalRelation.newBuilder()