Add Support to Limit the Maximum Depth of a Query

[ThisaruGuruge]

GraphQL service should be able to provide a way to limit the depth of an accepted query, so that too deep queries will not be executed, but invalidated at the validation phase.

Consider the following example, where there is a cyclic reference between the Book and the Author records.

import ballerina/graphql;

graphql:ListenerConfiguration configs = { maxQueryDepth: 3 }
listener graphql:Listener graphqlListener = new(9090, configs);

service /graphql on graphqlListener {
    isolated resource function get book(int id) returns Book {
        // Get Book record from somewhere and return
    }
}

type Book record {|
    string title;
    Author author;
|};

type Author record {|
    string name;
    Book[] books;
|};

Someone can exploit this cyclic reference to send a too deep query in order to break the service as below:

{
    book(id: 1) {
        author {
            books {
                author {
                    books {
                        author {
                            books {
                                author {
                                    // This can go on like this forever
                                    name
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

Therefore developers should be able to limit the maximum depth for a query to be valid, before execute it.

Subtask of #710