Add candidates into a project.

Returns 204 no content if candidates are successfully added to the project.

If candidates are already in the project, a 400 error will be returned with errors containing information on the candidates already in the project in this shape:

{
    "code": 400,
    "errors": {
        "json": {
            "candidate_ids": [
                "string"
            ]
        }
    },
    "message": "Candidates with the ids are already in the project",
    "status": "Bad Request"
}