[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.py

@@ -0,0 +1,12 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: NamespaceB
+
+
+def UnionInNestedNSCreator(unionType, table):
+    from flatbuffers.table import Table
+    if not isinstance(table, Table):
+        return None
+    if unionType == UnionInNestedNS().TableInNestedNS:
+        return TableInNestedNST.InitFromBuf(table.Bytes, table.Pos)
+    return None