feat(api): endpoints to mark and unmark custom records

internal/api/api_test.go

@@ -382,7 +382,7 @@ func TestChangesetResponseEmptyMarshalsToArrays(t *testing.T) {
 		t.Fatal(err)
 	}
 	s := string(b)
-	for _, want := range []string{`"updates":[]`, `"prunes":[]`, `"readOnly":[]`} {
+	for _, want := range []string{`"updates":[]`, `"prunes":[]`, `"customs":[]`, `"readOnly":[]`} {
 		if !strings.Contains(s, want) {
 			t.Fatalf("expected %s in %s", want, s)
 		}