Develop new service for proxying calls for Moodle mod_assign webservice functions

Required endpoints:

• Get assignments of Moodle course GET /services/moodle/api/assignments?course_id={course_id}
• Create new assignment in Moodle course POST /services/moodle/api/assignments?course_id={course_id}
• Delete assignment in Moodle course POST /services/moodle/api/assignments/{assignment_id}
• Submit new file to assignment in Moodle course POST /services/moodle/api/assignments/{assignment_id}/submissions
• Get all submissions for an assignment in Moodle course GET /services/moodle/api/assignments/{assignment_id}/submissions
• Grade submissions for an assignment in Moodle course POST /services/moodle/api/assignments/{assignment_id}/grade

These endpoints use the Moodle webservice functions of the mod_assign module: https://docs.moodle.org/dev/Web_service_API_functions