[Backend] - 355 submission repository get method

[brieromb]

Added 2 methods in the submission repository:

public abstract getAllForStudentInAssignmentStep(
    studentId: string,
    assignmentId: string,
    learning_object_id: string,
): Promise<Submission[]>;

-> gets all the submissions that a student made for a certain step in an assignment.

public abstract getByStudentId(studentId: string): Promise<Submission[]>;

-> gets all the submissions that a student made for any step in any assignment.