Mentor details - add project

[edmondpr]

Design: https://www.figma.com/file/DgrKZX9mS13Zgf7k5pZ1WT/Partner-web-app?type=design&node-id=727-148&t=LPfyjEb31x9adSCp-4

When clicking on the Add project button, the logged in user will be able to add for the selected mentor only one project that has no course associated to it, from the list of available projects (the ones that are not already associated with existing courses).

The /api/v1/partners/{partner_id}/mentors/{mentor_id}/projects endpoint will be used for retrieving the list of available projects and for adding the project.

[ugudlado]

@edmondpr How is this different from this endpoint to create project: #56 ?

or do we need same behavior for this endpoint as well?