[KYUUBI #758] [KYUUBI 661] Add UDF system_user

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### _Why are the changes needed?_
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add udf system_user.

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [ ] [Run test](https://kyuubi.readthedocs.io/en/latest/tools/testing.html#running-tests) locally before make a pull request

Closes #758 from zhaomin1423/661_add-system_user.

Closes #758

70eba56a [Min Zhao] update system user name
f5edc621 [Min Zhao] [KYUUBI 661] Add UDF system_user

Authored-by: Min Zhao <zhaomin1423@163.com>
Signed-off-by: Kent Yao <yao@apache.org>

docs/sql/functions.md

@@ -13,4 +13,5 @@ Kyuubi provides several auxiliary SQL functions as supplement to Spark's [Built-
 Name | Description | Return Type | Since
 --- | --- | --- | ---
 kyuubi_version | Return the version of Kyuubi Server | string | 1.3.0
+system_user | Return the system user name for the associated query engine | string | 1.3.0
 

externals/kyuubi-spark-sql-engine/src/main/scala/org/apache/kyuubi/engine/spark/udf/KDFRegistry.scala

@@ -36,6 +36,13 @@ object KDFRegistry {
     "string",
     "1.3.0")
 
+  val system_user: KyuubiDefinedFunction = create(
+    "system_user",
+    udf(() => System.getProperty("user.name")).asNonNullable(),
+    "Return the system user name for the associated query engine",
+    "string",
+    "1.3.0")
+
   def create(
     name: String,
     udf: UserDefinedFunction,

kyuubi-common/src/test/scala/org/apache/kyuubi/operation/JDBCTests.scala

@@ -359,4 +359,12 @@ trait JDBCTests extends BasicJDBCTests {
       assert(rs.getString(1) == KYUUBI_VERSION)
     }
   }
+
+  test("kyuubi defined function - system_user") {
+    withJdbcStatement() { statement =>
+      val rs = statement.executeQuery("SELECT system_user()")
+      assert(rs.next())
+      assert(rs.getString(1) == System.getProperty("user.name"))
+    }
+  }
 }