Add strict_date_optional_time_nanos field type.

Original Pull Request #2684
Closes #2676

Author: sothawo

src/main/java/org/springframework/data/elasticsearch/annotations/DateFormat.java

@@ -48,6 +48,7 @@ public enum DateFormat {
 	date_hour_minute_second_fraction("uuuu-MM-dd'T'HH:mm:ss.SSS"), //
 	date_hour_minute_second_millis("uuuu-MM-dd'T'HH:mm:ss.SSS"), //
 	date_optional_time("uuuu-MM-dd['T'HH:mm:ss.SSSXXX]"), //
+	strict_date_optional_time_nanos("uuuu-MM-dd['T'HH:mm:ss.SSSSSSXXX]"), //
 	date_time("uuuu-MM-dd'T'HH:mm:ss.SSSXXX"), //
 	date_time_no_millis("uuuu-MM-dd'T'HH:mm:ssVV"), // here Elasticsearch uses the zone-id in its implementation
 	epoch_millis("epoch_millis"), //

src/test/java/org/springframework/data/elasticsearch/core/convert/ElasticsearchDateConverterUnitTests.java

@@ -274,6 +274,12 @@ void shouldConvertDateOptionalTime() {
 		check(ElasticsearchDateConverter.of(DateFormat.date_optional_time), LocalDateTime.class);
 	}
 
+	@Test // #2676
+	@DisplayName("should convert strict_date_optional_time_nanos")
+	void shouldConvertStrictDateOptionalTime() {
+		check(ElasticsearchDateConverter.of(DateFormat.strict_date_optional_time_nanos), LocalDateTime.class);
+	}
+
 	@Test // #1647
 	@DisplayName("should convert date_time")
 	void shouldConvertDateTime() {