add support for findSingleAttributeSet

ebean-api/src/main/java/io/ebean/ExpressionList.java

@@ -383,6 +383,15 @@ public interface ExpressionList<T> {
    */
   <A> List<A> findSingleAttributeList();
 
+  /**
+   * Executes the query returning a set of values for a single property.
+   *
+   * This can be used to cache sets.
+   *
+   * @return a HashSet of values for the selegted property
+   */
+  <A> Set<A> findSingleAttributeSet();
+
   /**
    * Execute a query returning a single value of a single property/column.
    * <pre>{@code

ebean-api/src/main/java/io/ebean/ExtendedServer.java

@@ -368,6 +368,11 @@ public interface ExtendedServer {
    */
   <A, T> List<A> findSingleAttributeList(Query<T> query, Transaction transaction);
 
+  /**
+   * Execute the query returning a hashset of values for a single property.
+   */
+  <A, T> Set<A> findSingleAttributeSet(Query<T> query, Transaction transaction);
+
   /**
    * Execute the query returning at most one entity bean or null (if no matching
    * bean is found).

ebean-api/src/main/java/io/ebean/Query.java

@@ -925,6 +925,11 @@ public interface Query<T> extends CancelableQuery {
    */
   <A> List<A> findSingleAttributeList();
 
+  /**
+   * Execute the query returning a hashset of values for a single property.
+   */
+  <A> Set<A> findSingleAttributeSet();
+
   /**
    * Execute a query returning a single value of a single property/column.
    * <p>