[doc][feature] add doc about this function, add timezone arg in from_…

…unixtime function and unit test

docs/en/transform-v2/sql-functions.md

@@ -848,6 +848,30 @@ Example:
 
 YEAR(CREATED)
 
+### FROM_UNIXTIME
+
+```FROM_UNIXTIME (unixtime, formatString,timeZone)```
+
+Convert the number of seconds from the UNIX epoch (1970-01-01 00:00:00 UTC) to a string representing the timestamp of that moment.
+
+The most important format characters are: y year, M month, d day, H hour, m minute, s second. For details of the format, see `java.time.format.DateTimeFormatter`.
+
+`timeZone` is optional, default value is system's time zone.  `timezone` value can be a `UTC+ timezone offset`, for example, `UTC+8` represents the Asia/Shanghai time zone, see `java.time.ZoneId`
+
+This method returns a string.
+
+Example:
+
+// use default zone
+
+CALL FROM_UNIXTIME(1672502400, 'yyyy-MM-dd HH:mm:ss') 
+
+or
+
+// use given zone
+
+CALL FROM_UNIXTIME(1672502400, 'yyyy-MM-dd HH:mm:ss','UTC+6')
+
 ## System Functions
 
 ### CAST

seatunnel-transforms-v2/src/main/java/org/apache/seatunnel/transform/sql/zeta/functions/DateTimeFunction.java

@@ -550,9 +550,13 @@ public static String fromUnixTime(List<Object> args) {
             return null;
         }
         String format = (String) args.get(1);
+        ZoneId zoneId = ZoneId.systemDefault();
+        if (args.size() == 3) {
+            String timeZone = (String) args.get(2);
+            zoneId = ZoneId.of(timeZone);
+        }
         DateTimeFormatter df = DateTimeFormatter.ofPattern(format);
-        LocalDateTime datetime =
-                Instant.ofEpochSecond(unixTime).atZone(ZoneId.systemDefault()).toLocalDateTime();
+        LocalDateTime datetime = Instant.ofEpochSecond(unixTime).atZone(zoneId).toLocalDateTime();
         return df.format(datetime);
     }
 }

seatunnel-transforms-v2/src/test/java/org/apache/seatunnel/transform/sql/zeta/DateTimeFunctionTest.java

@@ -38,21 +38,28 @@ public void testFromUnixtimeFunction() {
                 new SeaTunnelRowType(
                         new String[] {"unixtime"}, new SeaTunnelDataType[] {BasicType.LONG_TYPE});
 
+        // 1672502400 means `2023-01-01 00:00:00` in unix time
+        Long unixTime = 1672502400L;
+        SeaTunnelRow inputRow = new SeaTunnelRow(new Long[] {unixTime});
+
         // transform by `from_unixtime` function
         sqlEngine.init(
                 "test",
                 null,
                 rowType,
                 "select from_unixtime(unixtime,'yyyy-MM-dd HH:mm:ss') as ts from test");
-
-        // 1672502400 means `2023-01-01 00:00:00` in unix time
-        Long unixTime = 1672502400L;
-        SeaTunnelRow inputRow = new SeaTunnelRow(new Long[] {unixTime});
-
-        // transform by sql
         SeaTunnelRow outRow = sqlEngine.transformBySQL(inputRow);
         Object field = outRow.getField(0);
-
         Assertions.assertEquals("2023-01-01 00:00:00", field.toString());
+
+        // transform by `from_unixtime` time zone function
+        sqlEngine.init(
+                "test",
+                null,
+                rowType,
+                "select from_unixtime(unixtime,'yyyy-MM-dd HH:mm:ss','UTC+6') as ts from test");
+        SeaTunnelRow outRow1 = sqlEngine.transformBySQL(inputRow);
+        Object field1 = outRow1.getField(0);
+        Assertions.assertEquals("2022-12-31 22:00:00", field1.toString());
     }
 }