Allow xmlName on union shapes

Because union shapes can be targeted by the httpPayload trait, we need
to allow xmlName on unions to alter the outermost wrapper that is used
to serialize the union when using a REST + XML protocol.

docs/source/spec/xml.rst

@@ -25,10 +25,10 @@ if necessary.
 Structure and union serialization
 =================================
 
-All XML serialization starts with a structure. The shape name of a structure
-is used as the outermost XML element name. Members of a structure or union
-shape are serialized as nested XML elements where the name of the element is
-the same as the name of the member.
+All XML serialization starts with a structure or union. The shape name of a
+structure/union is used as the outermost XML element name. Members of a
+structure/union are serialized as nested XML elements where the name of the
+element is the same as the name of the member.
 
 For example, given the following:
 
@@ -61,11 +61,11 @@ The :ref:`xmlattribute-trait` is used to serialize a structure
 member as an XML attribute.
 
 
-``xmlName`` on structures
--------------------------
+``xmlName`` on structures and unions
+------------------------------------
 
-An ``xmlName`` trait applied to a structure changes the element name of the
-serialized structure; however, it does not influence the serialization of
+An ``xmlName`` trait applied to a structure or union changes the element name
+of the serialized shape; however, it does not influence the serialization of
 members that target it. Given the following:
 
 .. tabs::
@@ -849,11 +849,12 @@ The XML serialization is:
 -----------------
 
 Summary
-    Changes the serialized element or attribute name of a structure or member.
+    Changes the serialized element or attribute name of a structure, union,
+    or member.
 Trait selector
-    ``:test(structure, member)``
+    ``:test(structure, union, member)``
 
-    *A structure or member*
+    *A structure, union, or member*
 Value type
     ``string`` value that MUST adhere to the :token:`xml_name` ABNF production:
 

smithy-model/src/main/resources/software/amazon/smithy/model/loader/prelude-traits.smithy

@@ -151,8 +151,8 @@ structure xmlAttribute {}
 @tags(["diff.error.const"])
 structure xmlFlattened {}
 
-/// Changes the serialized element or attribute name of a structure or member.
-@trait(selector: ":test(structure, member)")
+/// Changes the serialized element or attribute name of a structure, union, or member.
+@trait(selector: ":test(structure, union, member)")
 @tags(["diff.error.const"])
 @pattern("^[a-zA-Z_][a-zA-Z_0-9-]*(:[a-zA-Z_][a-zA-Z_0-9-]*)?$")
 string xmlName