[FEATURE] send_message_public_channel metadata event payload should accept either a string or JSON object

[schisamo]

Is your feature request related to a problem? Please describe. Full details on #4145

Describe the solution you'd like The Metadata Event Payload input property on the send_message_public_channel step accepts both a string or JSON object.

Do you have a workaround? Yes. JSON.stringify can be used to convert the JSON object to a string.