[Cosmos] custom transport prep work

[simorenoh]

Follow-up to #37419, which I had previously closed.

As part of the HTTP/2 readiness for the SDK, in order to handle custom transports, there was a couple things we had to do:

• make sure we can deal with response.content, which will be the new way to process the response data
• remove areas where we are manually setting values into response headers, since the possibility exists of now receiving immutable response headers due to HTTP/2's performance optimizations

With these changes we ensure our SDK is HTTP/2 ready.

Also took the opportunity to update our logs to reflect what it should actually have looked like for the logger setup to work as expected when used on a per-request basis - just having the logger named azure would make it a parent that would log all of the requests due to the policy's call to super.__init__().

[azure-sdk]

API change check

API changes are not detected in this pull request.