Add a default `application/xml` serialization library

[andrueastman]

Looks like some endpoints on Graph support application/xml content type now. e.g https://learn.microsoft.com/en-us/graph/api/trustframeworkpolicy-get?view=graph-rest-beta

To provide a better customer experience, we should add an xml serialization library that is registered with other serializers too.

Related to https://github.com/microsoftgraph/msgraph-beta-sdk-dotnet/issues/883