Add getZone to JodaCompatibleZonedDateTime (#37084)

The ZonedDateTime#getZone() was not accessible via the Joda shim. This
commit adds getZone() and exposes it through painless.

modules/lang-painless/src/main/resources/org/elasticsearch/painless/spi/org.elasticsearch.txt

@@ -87,6 +87,7 @@ class org.elasticsearch.script.JodaCompatibleZonedDateTime {
   int getNano()
   int getSecond()
   int getYear()
+  ZoneId getZone()
   ZonedDateTime minus(TemporalAmount)
   ZonedDateTime minus(long,TemporalUnit)
   ZonedDateTime minusYears(long)

server/src/main/java/org/elasticsearch/script/JodaCompatibleZonedDateTime.java

@@ -151,6 +151,10 @@ public class JodaCompatibleZonedDateTime {
         return dt.getYear();
     }
 
+    public ZoneId getZone() {
+        return dt.getZone();
+    }
+
     public ZonedDateTime minus(TemporalAmount delta) {
         return dt.minus(delta);
     }

server/src/test/java/org/elasticsearch/script/JodaCompatibleZonedDateTimeTests.java

@@ -152,6 +152,10 @@ public class JodaCompatibleZonedDateTimeTests extends ESTestCase {
         assertThat(javaTime.getYear(), equalTo(jodaTime.getYear()));
     }
 
+    public void testZone() {
+        assertThat(javaTime.getZone().getId(), equalTo(jodaTime.getZone().getID()));
+    }
+
     public void testMillis() {
         assertMethodDeprecation(() -> assertThat(javaTime.getMillis(), equalTo(jodaTime.getMillis())),
             "getMillis()", "toInstant().toEpochMilli()");