add User Service

[JeanBarriere]

We need to create an User service to manage the user

We need the following methods:

• login
• register
• getUser -- needs a User Class obv.
• isUser -- check if there is a user in cache
• saveUser -- save the user data in the cache
• updateUser -- update the user with the Api Service
• removeUser | logout -- remove the user from the cache
• fetchUser -- fetch the user data from the Api and save it in the cache