[BE] Implement the Show Interest In a Project Idea Proposal API

[davorminchorov]

Business Requirements

As a member, I want to be able to show interest in a project idea proposal so that I can be notified when the project idea proposal gets accepted and start working on it.

Acceptance Criteria

• There should be an endpoint which will allow members to show interest in a proposed idea for a project
• When an idea is Accepted, the members who has shown interest in the idea should receive a notification (email, in-app, depending on their notification settings).
• This endpoint should require authentication
• There should be authorization guards to prevent other members from showing interest in the project idea for a specific member.
• There should be an API documentation for the endpoint