Support for Multiple Drupal Backends

@Deciphered, practically couldn't this be handled by having multiple instance of client created for the said purpose?

There has to be some sort of session support (thinking about authentication) that needs to be managed for a client to be really useful. Otherwise we limit the client to interact as anonymous user or rely on being served in such a way that cookie based session is carried on.

Some follow up slack discussion:

@brianperry
> Interesting idea. So rather than having multiple instances of the client, the idea here would be to combine data from all backends in a single instance? Get me all articles from all of my backends for example?

@Deciphered
> One example is a Decoupled content editing application, the ability to connect to multiple backends at the same time offers the ability to drag and drop content from one backend to another.
Alternatively, the multiple data sources could also be rendered side by side on the same frontend website.

Re:
> @Deciphered, practically couldn't this be handled by having multiple instance of client created for the said purpose?

It should definitely be possible to solve this manually with multiple instances of the client, but for cases where this is important having some built in functionality around this could be valuable. Thinking of things like:

* Being able to provide a backend id as an argument when performing client operations.
* Being able to retrieve cached content by id, regardless of backend.

Does seem like an enhancement for a future phase though.

Marking this as postponed as we focus on the vertical slice POC.