Implement WebAPI endpoint for CRUD on core configuration data

[ericerway]

This issue is for the following packages:

[ ] venia-concept
[ ] pwa-buildpack
[ ] peregrine
[X] pwa-module (or new package)
[ ] pwa-devdocs

This issue is a:

[ ] Bug
[] Feature suggestion
[ ] Documentation issue
[X] Other (Please Specify)

Environment

Question	Answer
Magento version
Operating System + version
node.js version (node -v)
npm version (npm -v)

Description

As a developer I want to configure a PWA project to connect to a Magento 2 store with the store configured correctly for PWA development and deployment so that I can get started and begin development quickly.

Expected result:

Rest API with OAUTH to query and mutate values in the core configuration data table and/or any system settings global to the Magento instance.
Testing and documentation (as appropriate)

Possible solutions:

[zetlen]

Relevant PRs:
magento/graphql-ce#126
magento/graphql-ce#130