Enum that represents the possible challenge frequency and
difficulty configurations for a web key.
Values:
CHALLENGE_SECURITY_PREFERENCE_UNSPECIFIED (0):
Default type that indicates this enum hasn't
been specified.
USABILITY (1):
Key tends to show fewer and easier
challenges.
BALANCE (2):
Key tends to show balanced (in amount and
difficulty) challenges.
SECURITY (3):
Key tends to show more and harder challenges.
