add zone string test

Author: diegomarquezp

java-core/google-cloud-core/src/main/java/com/google/cloud/Timestamp.java

@@ -55,7 +55,7 @@ public final class Timestamp implements Comparable<Timestamp>, Serializable {
       new DateTimeFormatterBuilder()
           .appendOptional(DateTimeFormatter.ISO_LOCAL_DATE_TIME)
           .optionalStart()
-          .appendOffset("+HH:MM", "Z")
+          .appendZoneOrOffsetId()
           .optionalEnd()
           .toFormatter()
           .withZone(ZoneOffset.UTC);

java-core/google-cloud-core/src/test/java/com/google/cloud/TimestampTest.java

@@ -251,6 +251,12 @@ void parseTimestampWithTimeZoneOffset() {
         .isEqualTo(Timestamp.parseTimestamp("2020-12-06T19:21:12.123+05:30"));
   }
 
+  @Test
+  void parseTimestampWithZoneString() {
+    assertThat(Timestamp.parseTimestampDuration("2020-12-06T08:51:12.123America/Toronto"))
+        .isEqualTo(Timestamp.ofTimeSecondsAndNanos(1607262672, 123000000));
+  }
+
   @Test
   void fromProto() {
     com.google.protobuf.Timestamp proto =