[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.kt

@@ -0,0 +1,14 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+package NamespaceA.NamespaceB
+
+@Suppress("unused")
+@ExperimentalUnsignedTypes
+class UnionInNestedNS private constructor() {
+    companion object {
+        const val NONE: UByte = 0u
+        const val TableInNestedNS: UByte = 1u
+        val names : Array<String> = arrayOf("NONE", "TableInNestedNS")
+        fun name(e: Int) : String = names[e]
+    }
+}