Feature: Add Xml library

[timotheeguerin]

Add xml library following the approved design.
fix #2970

Only minor change was inverting the parmaeters for @ns(namespace, prefix instead of @namespace(prefix, namespace) which makes it clearer with the other overload @ns(namespace: EnumMember)

[azure-sdk]

All changed packages have been documented.

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/prs/3035/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/prs/3035/

[markcowl]

A few minor nits.

[mpodwysocki]

@timotheeguerin do we have any plans for CDATA support or would that be intrinsic to the encoding and decoding to decide whether one belongs? For example, Notification Hubs stores various things in CDATA sections such as template bodies and other information. Or is that a follow on item to this?

[timotheeguerin]

@timotheeguerin do we have any plans for CDATA support or would that be intrinsic to the encoding and decoding to decide whether one belongs? For example, Notification Hubs stores various things in CDATA sections such as template bodies and other information. Or is that a follow on item to this?

Yeah this was left as a follow up, or as a serializer job. Current goal was to get parity with openapi for now. If you have some particular idea in mind on how specifying some data info would cover some use cases I would file a new issue with the example.