[FLINK-6068] [table] Support If() as a built-in function.

This cloes #3553.

flink-libraries/flink-table/src/main/scala/org/apache/flink/table/validate/FunctionCatalog.scala

@@ -161,6 +161,7 @@ object FunctionCatalog {
     "isFalse" -> classOf[IsFalse],
     "isNotTrue" -> classOf[IsNotTrue],
     "isNotFalse" -> classOf[IsNotFalse],
+    "if" -> classOf[If],
 
     // aggregate functions
     "avg" -> classOf[Avg],

flink-libraries/flink-table/src/test/scala/org/apache/flink/table/expressions/ScalarOperatorsTest.scala

@@ -154,6 +154,11 @@ class ScalarOperatorsTest extends ExpressionTestBase {
       "true.?(true.?(true.?(10, 4), 4), 4)",
       "10")
     testTableApi(true, "?((f6 && true), 'true', 'false')", "true")
+    testTableApi(
+      If('f9 > 'f8, 'f9 - 1, 'f9),
+      "If(f9 > f8, f9 - 1, f9)",
+      "9"
+    )
     testSqlApi("CASE 11 WHEN 1 THEN 'a' ELSE 'b' END", "b")
     testSqlApi("CASE 2 WHEN 1 THEN 'a' ELSE 'b' END", "b")
     testSqlApi(