Creates a new user session.

When creating a session the application will ensure that the user .fmu directory exists by creating it if it does not.

If a valid session already exists when POSTing to this route, the existing session will be renewed with a new expiry date and a new session cookie value.

After creating the session, the application will attempt to find the nearest project .fmu directory above the current working directory and add it to the session if found. If not found, no project will be associated. Renewing an existing session keeps its current state.

The session cookie set by this route is required for all other routes. Sessions are not persisted when the API is shut down.