[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_test1.fbs

@@ -5,6 +5,10 @@ table TableInNestedNS
     foo:int;
 }
 
+union UnionInNestedNS {
+    TableInNestedNS,
+}
+
 enum EnumInNestedNS:byte
 {
 	A, B, C