Methods to restrict access to a URL based on login status.
Values:
LOGIN_UNSPECIFIED (0):
Not specified. LOGIN_OPTIONAL is assumed.
LOGIN_OPTIONAL (1):
Does not require that the user is signed in.
LOGIN_ADMIN (2):
If the user is not signed in, the auth_fail_action is
taken. In addition, if the user is not an administrator for
the application, they are given an error message regardless
of auth_fail_action. If the user is an administrator,
the handler proceeds.
LOGIN_REQUIRED (3):
If the user has signed in, the handler proceeds normally.
Otherwise, the auth_fail_action is taken.
