HSEARCH-3958 Make AggregationDescriptors singletons

Like we did some time ago with field type descriptors.

integrationtest/backend/tck/src/main/java/org/hibernate/search/integrationtest/backend/tck/metamodel/IndexValueFieldTypeDescriptorBaseIT.java

@@ -180,7 +180,7 @@ private boolean isSortSupported() {
 	}
 
 	private boolean isAggregationSupported() {
-		return new TermsAggregationDescriptor().getSingleFieldAggregationExpectations( fieldType ).isSupported();
+		return TermsAggregationDescriptor.INSTANCE.getSingleFieldAggregationExpectations( fieldType ).isSupported();
 	}
 
 	private class IndexBinding {

integrationtest/backend/tck/src/main/java/org/hibernate/search/integrationtest/backend/tck/search/aggregation/RangeAggregationSpecificsIT.java

@@ -67,7 +67,7 @@ public static Object[][] parameters() {
 		supportedFieldTypes = new LinkedHashSet<>();
 		dataSets = new ArrayList<>();
 		List<Object[]> parameters = new ArrayList<>();
-		AggregationDescriptor aggregationDescriptor = new RangeAggregationDescriptor();
+		AggregationDescriptor aggregationDescriptor = RangeAggregationDescriptor.INSTANCE;
 		for ( FieldTypeDescriptor<?> fieldType : FieldTypeDescriptor.getAll() ) {
 			if ( aggregationDescriptor.getSingleFieldAggregationExpectations( fieldType ).isSupported() ) {
 				supportedFieldTypes.add( fieldType );

integrationtest/backend/tck/src/main/java/org/hibernate/search/integrationtest/backend/tck/search/aggregation/TermsAggregationSpecificsIT.java

@@ -66,7 +66,7 @@ public static Object[][] parameters() {
 		supportedFieldTypes = new LinkedHashSet<>();
 		dataSets = new ArrayList<>();
 		List<Object[]> parameters = new ArrayList<>();
-		AggregationDescriptor aggregationDescriptor = new TermsAggregationDescriptor();
+		AggregationDescriptor aggregationDescriptor = TermsAggregationDescriptor.INSTANCE;
 		for ( FieldTypeDescriptor<?> fieldType : FieldTypeDescriptor.getAll() ) {
 			if ( aggregationDescriptor.getSingleFieldAggregationExpectations( fieldType ).isSupported() ) {
 				supportedFieldTypes.add( fieldType );

integrationtest/backend/tck/src/main/java/org/hibernate/search/integrationtest/backend/tck/testsupport/operations/AggregationDescriptor.java

@@ -22,8 +22,8 @@ public abstract class AggregationDescriptor {
 	public static List<AggregationDescriptor> getAll() {
 		if ( all == null ) {
 			all = Collections.unmodifiableList( Arrays.asList(
-					new RangeAggregationDescriptor(),
-					new TermsAggregationDescriptor()
+					RangeAggregationDescriptor.INSTANCE,
+					TermsAggregationDescriptor.INSTANCE
 			) );
 		}
 		return all;

integrationtest/backend/tck/src/main/java/org/hibernate/search/integrationtest/backend/tck/testsupport/operations/RangeAggregationDescriptor.java

@@ -35,6 +35,11 @@
 
 public class RangeAggregationDescriptor extends AggregationDescriptor {
 
+	public static RangeAggregationDescriptor INSTANCE = new RangeAggregationDescriptor();
+
+	private RangeAggregationDescriptor() {
+	}
+
 	@Override
 	public <F> ExpectationsAlternative<
 					SupportedSingleFieldAggregationExpectations<F>,

integrationtest/backend/tck/src/main/java/org/hibernate/search/integrationtest/backend/tck/testsupport/operations/TermsAggregationDescriptor.java

@@ -37,6 +37,11 @@
 
 public class TermsAggregationDescriptor extends AggregationDescriptor {
 
+	public static TermsAggregationDescriptor INSTANCE = new TermsAggregationDescriptor();
+
+	private TermsAggregationDescriptor() {
+	}
+
 	@Override
 	public <F> ExpectationsAlternative<
 					SupportedSingleFieldAggregationExpectations<F>,