A contact's validation state indicates whether or not it is
the correct contact to be receiving notifications for a
particular resource.
Values:
VALIDATION_STATE_UNSPECIFIED (0):
The validation state is unknown or
unspecified.
VALID (1):
The contact is marked as valid. This is
usually done manually by the contact admin. All
new contacts begin in the valid state.
INVALID (2):
The contact is considered invalid. This may
become the state if the contact's email is found
to be unreachable.
