Move request methods to pipeline

[heaths]

Rather than expose the underlying HttpClient, we should move request methods to the pipeline like in our other SDKs. We have helper methods for creating paged results, handling different error cases, etc. This also make it possible to abstract the pipeline from the protocol and handle, potentially, AMQP.

[yoshuawuyts]

Can we merge this issue into the #290 tracking issue?

[heaths]

That's all rather specific to Cosmos. This is specifically about refactoring the pipeline now so that we're not depending on reqwest's Request (or wherever it comes from - I forget off hand).

[cataggar]

I want to update the services to do this.

[cataggar]

Services have been updated in #527.

[cataggar]

Closing this in favor of #802.