Consider switching to azure.core.rest.HttpResponse for version-tolerant=False code gen

[jalauzon-msft]

Currently autorest will generate code which produces responses of type azure.core.pipeline.transport.HttpResponse if version-tolerant=False. This namespace is being deprecated and PyLint rules are being introduced to generate errors if this import is used.

Autorest should consider switching to azure.core.rest.HttpResponse even for non-version-tolerant packages as the new class should be fully backwards compatible and therefore it may not be a breaking change to switch these return types. Some testing should probably be done to ensure this is the case though. Note: I autorest has already switched the requests to azure.core.rest.HttpRequest a while back.

CC: @iscai-msft