Make GraphQL service work with GraphQL Playground

[shafreenAnfar]

GraphQL Playground is a popular tool for working with GraphQL services.

This tool can be used to interact with Ballerina GraphQL services. But following things do not work.

1. Client side query validation and suggestions
2. Schema is not retrieved
3. Documentation is not retrieved

We need to start working on supporting each in order.

[shafreenAnfar]

It seems the first point is failing because GraphQL service could not handle the below introspection query.

query IntrospectionQuery {
    __schema {
        queryType {
            name
        }
        mutationType {
            name
        }
        subscriptionType {
            name
        }
        types {
            ...FullType
        }
        directives {
            name
            description
            locations
            args {
                ...InputValue
            }
        }
    }
}
        
fragment FullType on __Type {
    kind
    name
    description
    fields(includeDeprecated: true) {
        name
        description
        args {
            ...InputValue
        }
        type {
            ...TypeRef
        }
        isDeprecated
        deprecationReason
    }
    inputFields {
        ...InputValue
    }
    interfaces {
        ...TypeRef
    }
    enumValues(includeDeprecated: true) {
        name
        description
        isDeprecated
        deprecationReason
    }
    possibleTypes {
        ...TypeRef
    }
}
        
fragment InputValue on __InputValue {
    name
    description
    type {
        ...TypeRef
    }
    defaultValue
}
        
fragment TypeRef on __Type {
    kind
    name
    ofType {
        kind
        name
        ofType {
            kind
            name
            ofType {
                kind
                name
                ofType {
                    kind
                    name
                    ofType {
                        kind
                        name
                        ofType {
                            kind
                            name
                            ofType {
                                kind
                                name
                            }
                        }
                    }
                }
            }
        }
    }
}