The purpose of this milestone is to define a backend set of APIs used by a frontend client to allow a user to login, establish a session, select what organization context is valid for the session and their role context (i.e. are they operating as a coach or coachee?).

Definition of Done:

1. Establish a user session store (hint: look at crates.io and other Axum examples)
2. A backend API exists that emulates the Phoenix backend version of the login system
3. A backend API exists that allows a user to set their current Organization
4. A backend API exists that allows a user to set their current CoachingRole context

Note:

1. One idea is to use the AppState struct as the interface and in-memory session context. The application would need to know when are key moments to update the persisted session store and it would do this by serializing AppState.