Sample for PendingGuard invoking an external service and waiting for callback by a Human Interaction

# Feature Request

Provide an example for pending guards with human interaction, i.e. a sample invoking a third party service while a pending guard ensures no state transition took place until a call back endpoint is called to reset the pending guard.

## Which Areas Would Be Affected?

none, as it is only a sample demonstrating currently available capabilities

## Why Is the Feature Desired?

poor documentation

## Who will sponsor this feature?

@ronjaquensel

## Solution Proposal

For Simpl I had written a contract Manager extension, that is exactely doing this. I will provide a documentated sample.