Design doc: access to configured clients

During acceptance testing, provider test code needs access to configured clients in order to make API requests.

The API client may be used for test setup/teardown, checking that API values were correctly updated, and other operations not covered by the resource CRUD functions.

This functionality could be made available to provider developers in a number of ways, including:
- Leave it up to them
- Add helpers for getting it off the configured provider
- Guide people towards instantiating a new client