feat: Send Message to Flow #146

Create a new Flow Designer custom action called “Send Message”, which allows one flow to send a message to another flow instance that is currently waiting at a “Wait for Message” condition.

This action enables asynchronous communication between flows by using the sn_fd.FlowAPI.sendMessage() method to deliver a message to a specified flow context. It is useful for building event-driven or distributed process logic where different flows need to signal each other’s progress or completion.

Inputs:
• context_id (String, required) — The context ID of the target flow waiting for a message.
• message (String, required) — The message payload to send.

Behavior:
When executed, the action should call the Flow API to send the provided message to the specified context ID:
sn_fd.FlowAPI.sendMessage(inputs.context_id, inputs.message);