[Recruitment] - createMilestone endpoint

[Aaryan1203]

Desired Changes

Blocked by #2699

We need to be able to create Milestones. To do this we will need an endpoint. It should be under the route /recruitment/timeline/create and should take in a date, milestone, and description. Only admins should be able to create milestones

Acceptance Criteria

• There is a new endpoint under the route /recruitment/milestone/create
• Only admins can create timelines
• Unit tests are added to ensure everything works correctly
• Screenshot of manual tests from postman

Screenshots (as needed)

No response