[DPG] Access to important response headers

[annatisch]

Currently autorest generated methods only allow access to the response headers via either:

• Passing in a raw_response_hook callable.
• Passing in cls for custom deserialization (not documented nor advised that end customers use this).

It's therefore difficult to access headers that are an important part of the response. For example, the "etag" returned from get_resource operation, as is the case for JobRouter SDK: https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/communication/azure-communication-jobrouter/azure/communication/jobrouter/_operations/_operations.py#L1256

We need a design to make accessing this feel more integrated into the scenario.

[lmazuel]

• Have a response_header that returns the raw dict of the headers. This is all the time.
• Expose as attribute at the root, named correctly, based on TypeSpec, if the model generation is on