[rest] add backcompat mixin to rest requests

[iscai-msft]

Right now when using our azure.core.rest.HttpRequest, we have to convert these to and from azure.core.pipeline.transport.HttpRequests. Additionally, they can't be used in our pipelines / pollers / pagers etc immediately.

The plan is to give our new requests the same surface design as our old requests, so all policies / transports can handle them without converting them to the old design. To do this, we want to add deprecated properties and methods to our new requests to give it the same shape as the old design. Make sure these properties don't show up in documentation, and try your best to hide them as much as possible