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 microsoftgraph/msgraph-beta-sdk-dotnet#883

[baywet]

$value is a "binary payload" by nature in odata conventions. Meaning it's not schematized. So even if we implemented a parsing/serialization provide for XML, the description would still not contain a schema for this operation. And kiota would still generate a Task or equivalent.

We could get around that by adding post processing to the OAS description, or something in XOD + implementation in Yoko, but that's probably a lot of work end to end for a single scenario at this point.

My recommendation here is that we leave this on ice until we get overwhelming customer demand for it.

Of course the other scenarios being:

• Graph can technically return xml instead of JSON for most structured endpoints if you ask nicely. But I'm not sure why anybody choose that when it's going to result in heavier payloads and when they already have a client using JSON.
• 3rd party APIs might be XML only. But so far nobody has requested support for it.