Add some ways to work with user_email in remove_project_collaborator() and update_project_collaborator()

When using remove_project_collaborator() or update_project_collaborator() one needs user_id which is not exactly easy to get. It can be extracted from list_project_collaborators() which provides the info, but it is not really handy or intuitive.

Either remove_project_collaborator() and update_project_collaborator() should take also user_email besides user_id to allow editing collaborators for project. Or there could be new Endpoint and client function to get user_id from user_email.

Comienza leyendo remove_project_collaborator(), update_project_collaborator() y list_project_collaborators() para entender cómo gestionan actualmente user_id. Aclara si el resultado previsto es añadir compatibilidad con email en las funciones existentes o crear un endpoint de búsqueda por user_id y, después, define el comportamiento de la API y las pruebas necesarias para demostrar que las actualizaciones de colaboradores funcionan con el enfoque elegido.