Create a route that lists interviewer stats

[weihanwang]

1. create a new route getInterviewerStats in file src/api/routes/interviews.ts
2. access to this route should be limited to MentorshipManagers only
3. It should return an array of users who has the role Mentor
4. The type of each array item should be like:

{
  user: zUser,
  interviews: z.number()
}

where interviews is the number of times the user is referred to in the InterviewFeedback.interviewerId field