Allow Intersection Types in GraphQL Services

[ThisaruGuruge]

Description:
Currently, GraphQL services cannot use intersection types (with readonly & T) as inputs. This should be allowed.

The following service should be valid.

import ballerina/graphql;

public type Person record {
    string name;
    int age;
};

public type Student record {
    string name;
    string subject;
};

public type Book readonly & record {
    string name;
    string author;
};

public type Movie readonly & record {
    string name;
    string director;
};

service /graphql on new graphql:Listener(4000) {

    // Intersection with a type reference as an input
    isolated resource function get name(Person & readonly person) returns string {
        return person.name;
    }

    // Intersection with a type reference as an output
    isolated resource function get subject() returns Student & readonly {
        Student student = {name: "Jesse Pinkman", subject: "Arts and Crafts"};
        return student.cloneReadOnly();
    }

    // type reference of an intersection type as an output
    isolated resource function get director() returns Movie {
        Movie movie = {name: "Pulp Fiction", director: "Quentin Tarantino"};
        return movie;
    }

    // type reference of an intersection type as an input
    isolated resource function get author(Book book) returns string {
        return book.author;
    }

    // Intersection with a type reference array as an input
    isolated resource function get names(Person[] & readonly people) returns string[] {
        return people.'map(person => person.name);
    }

    // Intersection with a type reference array as an output
    isolated resource function get subjects() returns Student[] & readonly {
        Student[] students = [{name: "Jesse Pinkman", subject: "Arts and Crafts"}];
        return students.cloneReadOnly();
    }

    // type reference of an intersection type array as an output
    isolated resource function get directors() returns Movie[] {
        Movie[] movies = [{name: "Pulp Fiction", director: "Quentin Tarantino"}];
        return movies;
    }

    // type reference of an intersection type array as an input
    isolated resource function get authors(Book[] books) returns string[] {
        return books.map(book => book.author);
    }
}