CLI for managing GraphQL query variables

[hatertron3000]

Create a CLI for adding, configuring, and removing GraphQL query variables. Adding a variable must:

• prompt the user to select an existing setting in schema.json to populate the variable
• declare the variable in the query in ./storefront_api_query.graphql
• create or update a hidden schema type in schema.json with a graphQueries setting. Example:

  {
    "type": "hidden",
    "settings": [
      {
        "type": "graphQl",
        "id": "graphQueries",
        "typeMeta": {
          "mappings": {
            "productId": {
              "reads": "productId",
              "type": "Int!"
            }
          }
        }
      }
    ]
  }

• create or update the mappings in the graphQueries setting's typeMeta