Started implementation for private flags in rust (#7269)

Adds flag to the cpp to fix bad annotations in all the languages

Addresses comments to the PR, and fixes logic for leaking annotations

tests/private_annotation_test.fbs

@@ -0,0 +1,18 @@
+enum AB:byte (private) {
+  A,
+  B,
+}
+
+struct Object (private) {
+    value: int;
+}
+
+union Any (private) { Game, Annotations }
+
+table Game (private) {
+    value: int;
+}
+
+table Annotations (private) {
+    value: int;
+}