[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.java

@@ -0,0 +1,14 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+package NamespaceA.NamespaceB;
+
+public final class UnionInNestedNS {
+  private UnionInNestedNS() { }
+  public static final byte NONE = 0;
+  public static final byte TableInNestedNS = 1;
+
+  public static final String[] names = { "NONE", "TableInNestedNS", };
+
+  public static String name(int e) { return names[e]; }
+}
+