feat: Introduce API endpoints and refactor service methods for adding, removing, listing, and setting primary course instructors.

Backend/src/types/CoursesInstructor.types.ts

@@ -101,6 +101,7 @@ export interface listCourseinstructorResponse {
 
 export interface addinstructorCourse {
     token: string;
+    user_id: number;
     course_id: number;
 }
 
@@ -119,6 +120,11 @@ export interface listinstructorCourseResponse {
     }[];
 }
 
+export interface listinstructorCourse {
+    token: string;
+    course_id: number;
+}
+
 export interface removeinstructorCourse {
     token: string;
     user_id: number;