[SPARK] Allow list timestampdiff & timestampadd as generated column expressions (#4048)

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#### Which Delta project/connector is this regarding?
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- [x] Spark
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## Description
It allow-lists `timestampdiff` & `timestampadd` as generated column
expressions as they are deterministic.
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## How was this patch tested?
New test case.
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## Does this PR introduce _any_ user-facing changes?
Previously users cannot use `timestampdiff` or `timestampadd` in
generation expression. The PR allows this.
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Author: zhipengmao-db

spark/src/main/scala/org/apache/spark/sql/delta/SupportedGenerationExpressions.scala

@@ -213,6 +213,8 @@ object SupportedGenerationExpressions {
     expression[Now]("now"),
     expression[Quarter]("quarter"),
     expression[Second]("second"),
+    expression[TimestampAdd]("timestampadd"),
+    expression[TimestampDiff]("timestampdiff"),
     expression[ParseToTimestamp]("to_timestamp"),
     expression[ParseToDate]("to_date"),
     // `gettimestamp` is not a Spark built-in class but `ParseToDate` will refer to

spark/src/test/scala/org/apache/spark/sql/delta/GeneratedColumnSuite.scala

@@ -1266,6 +1266,18 @@ trait GeneratedColumnSuiteBase
     }
   }
 
+  test("generation expression allows timestampdiff & timestampadd") {
+    withTableName("generation_expression_timestamp_diff_add") { tableName =>
+      createTable(
+        tableName,
+        path = None,
+        schemaString = "c1 TIMESTAMP, c2 TIMESTAMP, c3 BIGINT, c4 TIMESTAMP",
+        generatedColumns =
+          Map("c3" -> "timestampdiff(MONTH, c1, c2)", "c4" -> "timestampadd(MONTH, 1, c1)"),
+        partitionColumns = Seq.empty)
+    }
+  }
+
   test("MERGE UPDATE basic") {
     withTableName("source") { src =>
       withTableName("target") { tgt =>