[Prototype] Support composite ObjectSerializers, add XmlSerializer

[alzimmermsft]

Description

Prototype adding support for composite ObjectSerializers which will allow for RestProxy, and other code locations, to be created with multiple serialization formats for cases where a service uses multiple serialization formats. Additionally, XmlSerializer is added, the XML equivalent for JsonSerializer for JSON.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Pull request includes test coverage for the included changes.