[GraphQL caching] Add support for queries via HTTP GET

[paliarush]

Based on the prototype and this commit specifically, add support for GraphQL queries via HTTP GET. Note that GraphQL mutations must still only be supported via HTTP POST.

Changes must be covered with GraphQL functional tests.
(This requires changes to the testing framework, which only supports post.)

[pcvonz]

Small note: Adding the ability to use GET would be great for caching client side via the Cache Storage API.

More info in this issue and this issue.

I'm currently working on caching for the Venia concept and the lack of this functionality has been one of the hurdles for a smooth implementation.