[fuzzer] Adds code generation target. (#8795)

* adds code generation fuzzer target.

* add buffer verification

* add table verification in codegen fuzzer

---------

Co-authored-by: Björn Harrtell <bjornharrtell@users.noreply.github.com>

tests/fuzzer/seed_codegen/test_union

@@ -0,0 +1,20 @@
+namespace Test;
+
+union Shape {
+  Circle,
+  Square,
+}
+
+table Circle {
+  radius: float;
+}
+
+table Square {
+  side: float;
+}
+
+table Root {
+  shape: Shape;
+}
+
+root_type Root;