[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.php

@@ -0,0 +1,23 @@
+<?php
+// automatically generated by the FlatBuffers compiler, do not modify
+
+namespace NamespaceA\NamespaceB;
+
+class UnionInNestedNS
+{
+    const NONE = 0;
+    const TableInNestedNS = 1;
+
+    private static $names = array(
+        UnionInNestedNS::NONE=>"NONE",
+        UnionInNestedNS::TableInNestedNS=>"TableInNestedNS",
+    );
+
+    public static function Name($e)
+    {
+        if (!isset(self::$names[$e])) {
+            throw new \Exception();
+        }
+        return self::$names[$e];
+    }
+}