Type of recognizer result being validated.
Contextual information containing the recognizer result and original options passed to the prompt.
The validator should be an asynchronous function that returns true if
prompt.recognized.value is valid and the prompt should end.
If the validator returns false the prompts default re-prompt logic will be run unless the
validator sends a custom re-prompt to the user using prompt.context.sendActivity(). In that
case the prompts default re-prompt logic will not be run.
Function signature for providing a custom prompt validator.