Feature Proposal: Allow custom headers in RESTful requests

At the moment, the client, either the high level or the low-level APIClient does not allow one to specify a custom header, only the base_url.

The reason I propose such a feature is that sometime an authentication header is required to access the endpoint. For example, say I use portainter to remotely manage daemons, then, I will be able to access all docker API via http://<portainer_url>/endpoints/<endpoint_id>/docker. However, it is not possible use this as the base_url with docker-py, because an authorization header is required to accompany it in each request.

By allowing user specified headers, then all features of docker-py can be used in such context. I am not entirely sure about the high-level API, but I do think the APIClient shall work as it is one-to-one mapping with Docker API.

Start by tracing how the low-level APIClient builds RESTful requests and how the high-level client delegates to it. Confirm the intended behavior for user-specified headers, including an authorization header, and determine whether both client layers should support them; done means custom headers accompany requests to the configured base_url.