Allow collapsing on inner hits

As described in
https://www.elastic.co/guide/en/elasticsearch/reference/7.9/collapse-search-results.html#second-level-of-collapsing,
collapsing is possible on the second level as well, on the inner hit.

Close olivere#1394
Close olivere#1401

inner_hit.go

@@ -41,6 +41,11 @@ func (hit *InnerHit) Query(query Query) *InnerHit {
 	return hit
 }
 
+func (hit *InnerHit) Collapse(collapse *CollapseBuilder) *InnerHit {
+	hit.source.Collapse(collapse)
+	return hit
+}
+
 func (hit *InnerHit) From(from int) *InnerHit {
 	hit.source.From(from)
 	return hit

inner_hit_test.go

@@ -42,3 +42,20 @@ func TestInnerHitWithName(t *testing.T) {
 		t.Errorf("expected\n%s\n,got:\n%s", expected, got)
 	}
 }
+
+func TestInnerHitSecondLevelCollapse(t *testing.T) {
+	hit := NewInnerHit().Name("by_location").Size(3).Collapse(NewCollapseBuilder("user.id"))
+	src, err := hit.Source()
+	if err != nil {
+		t.Fatal(err)
+	}
+	data, err := json.Marshal(src)
+	if err != nil {
+		t.Fatalf("marshaling to JSON failed: %v", err)
+	}
+	got := string(data)
+	expected := `{"collapse":{"field":"user.id"},"name":"by_location","size":3}`
+	if got != expected {
+		t.Errorf("expected\n%s\n,got:\n%s", expected, got)
+	}
+}