Add config option to disable the REST client

[lizkenyon]

Description

Fixes Disable REST client for new apps

• We want new apps created with the Ruby template to use the GraphQL API, and not the REST API
• After this is released we will enable this config option in the template
• By default this is false, apps currently using the REST API will not be affected
• If the config option is true, an error will be raised if REST client is used.

How has this been tested?

• Tested in a local development app.

disable-rest.mp4

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

[andy-liuu]

Code looks good! a quick question:

• should we be more forceful in disabling rest? (either now or in the future?)
  • i.e. remove rest entirely in a pr and call it a major release to the shopify_api gem. I think existing apps are unaffected this way too