[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/NamespaceA/NamespaceB/UnionInNestedNS.lua

@@ -0,0 +1,10 @@
+-- automatically generated by the FlatBuffers compiler, do not modify
+
+-- namespace: NamespaceB
+
+local UnionInNestedNS = {
+    NONE = 0,
+    TableInNestedNS = 1,
+}
+
+return UnionInNestedNS -- return the module