EventSource filter input

[erikperkins]

From the docs, it's not really clear what the expression key has access to. In the example, body.name obviously refers to the structure of a Kafka message, but what about the JSON payload of a generic webhook? It could be something like response.json.key or apple.banana.key.subkey.subsubkey, etc. It would be better to not have to guess. Is there a way to determine what variables can be referenced in a filter expression for a given EventSource?