GET requests support for queries

[rogyar]

Description (*)

This PR introduces a possibility to use GET requests for queries.

Fixed Issues (if relevant)

1. [GraphQL caching] Add support for queries via HTTP GET #229: Add support for queries via HTTP GET
2. GraphQL not working with GET #269: GraphQL not working with GET

Manual testing scenarios (*)

1. Switch HTTP request type from POST to GET
2. Use any query for testing the response

{
  products(filter: {category_id: {eq: "2"}}, pageSize: 2) {
    total_count
  }
}

[ghost]

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