[Rust] Ensure unions are referenced with the correct path (#6422)

* Add codegen test for namespaced unions

* [Rust] Handle cross-namespace union use

* [Rust] Test namespace handling

* [Rust] Drop trailing whitespace in codegen

* [Rust] Set flags in generate_code.bat to match .sh

* [C#] Add additional namespace test file

tests/namespace_test/namespace_test2.fbs

@@ -6,6 +6,7 @@ table TableInFirstNS
 {
     foo_table:NamespaceB.TableInNestedNS;
 	foo_enum:NamespaceB.EnumInNestedNS;
+    foo_union:NamespaceB.UnionInNestedNS;
 	foo_struct:NamespaceB.StructInNestedNS;
 }