graphql-client: Routing Support

Problem/Motivation

Similar to getResourceByPath on JsonApiClient, it should possible to resolve an entity by path and then query the resulting data.

Proposed resolution

If this can be achieved with standard GraphQL implementations (ie not a heavily customized schema,) we should document the approach and won't need to add new functionality.

If not, we could add a new method that first queries decoupled router to get an ID, and then performs a graphql query with that information.
* Should we actually create a method here, or should we just document how someone could use the DecoupledRouterClient and GrapqlClient together?

Remaining tasks

* Research possible solutions.
* Implement feature (if necessary)
* Add test coverage (if necessary)
* Update documentation.

API changes

TBD