Add changelog

Author: landonxjames

.changelog/unknown-debug.md

@@ -0,0 +1,18 @@
+---
+applies_to: ["client"]
+authors: ["landonxjames"]
+references: ["smithy-rs#4137"]
+breaking: false
+new_feature: false
+bug_fix: true
+---
+
+Fix bug with enum codegen
+
+When the first enum generated has the `@sensitive` trait the opaque type
+underlying the `UnknownVariant` inherits that sensitivity. This means that
+it does not derive `Debug`. Since the module is only generated once this
+causes a problem for non-sensitive enums that rely on the type deriving
+`Debug` so that they can also derive `Debug`. We manually add `Debug` to
+the module so it will always be there since the `UnknownVariant` is not
+modeled and cannot be `@sensitive`.