Values:
PARAMETER_STATE_UNSPECIFIED (0):
Not specified. This value should be never
used.
EMPTY (1):
Indicates that the parameter does not have a
value.
INVALID (2):
Indicates that the parameter value is
invalid. This field can be used by the webhook
to invalidate the parameter and ask the server
to collect it from the user again.
FILLED (3):
Indicates that the parameter has a value.