Add GraphQL support

[jcamiel]

This PR closes #504 : we support now GraphQL queries, with/without variables.

The syntax choosen is:

• Unnamed query:

POST http://localhost:8000/graphql
```graphql
{
  allFilms {
    films {
      title
      director
      releaseDate
    }
  }
}
```

Any GraphQL queries should be supported without issues.

• Query with variables:

POST http://localhost:8000/graphql
```graphql
query Person($id: ID!) {
  person(id: $id) {
    name
  }
}

variables {
  "id": "cGVvcGxlOjQ="
}
```

Variables is a JSON object, that can also be templatized with Hurl variables (à la Inception):

POST http://localhost:8000/graphql
[Options]
variable: id=cGVvcGxlOjQ=
```graphql
query Person($id: ID!) {
  person(id: $id) {
    name
  }
}

variables {
  "id": "{{id}}"
}
```

For the moment, we haven't yet addressed injecting GraphQL queries from files but we have a syntax in mind that will be elegant and simple without introducing new concept!

[fabricereix]

/accept

[hurl-bot]

🕗 /accept is running, please wait for completion.

[hurl-bot]

✅ Pull request accepted and closed by fabricereix with fast forward merge..

# List of commits merged from Orange-OpenSource/hurl/feature/graphql-variables branch into Orange-OpenSource/hurl/master branch:

• ed3217c Add support for GraphQL variables.