Update Autorest C# - network

[pakrym]

Contributes to #13511

Major features

1. Collections are now always initialized and collection properties are readonly by default
2. Internal deserialization ctors are removed for input-only models
3. Improved nullability support - fewer extra null checks, correct nullable types generated. Might require adding missing annotations to the swagger spec.
4. Improved header support - descriptions and x-ms-client-name honored.
5. Single value, modelAsString enums are generated as types, they were string constants before.
6. Readonly properties are not serialized - affects recordings.

What you might need to do:

1. If you copied and overrode the serialization code you might want to update it to be in sync with new generator patterns.
2. Please follow up on updating the centralized swagger file if there were workarounds applied to autorest.md

[allenjzhang]