The `context-provider-command` allows user to specify a command which will be run whenever the hook gets matched.
Webhook will pass the command a JSON string via the STDIN containing the request context (matched hook id, method used to trigger the hook, remote address, requested host, requested URI, raw body, headers and query values).
The output of the command must be a valid JSON string which will be mapped back into a special source named `context` that can be used with existing rules and directives.