Add a sketch of a channel API for simple use cases in other projects

shared/channels/amqp_helpers.py

-from typing import Callable
+from typing import Callable, Any
 
 import pika
+from marshmallow import Schema
 from pycapo import CapoConfig
+from workspaces.json import WorkflowEventSchema
 
 
 def make_amqp_connection(profile: str) -> pika.BlockingConnection:
@@ -82,3 +84,41 @@ def receive_amqp_message(
     channel.queue_bind(queue=queue_name, exchange=exchange, routing_key=routing_key)
     channel.basic_consume(queue=queue_name, on_message_callback=callback, auto_ack=auto_ack)
     channel.start_consuming()
+
+
+class Channel:
+    def __init__(self, schema: Schema):
+        self.schema = schema
+
+    def connect(self):
+        # FIXME implement this
+        pass
+
+    def close(self):
+        # FIXME implement this
+        pass
+
+    def send(self, event):
+        # I don't know how to type Event there for Python but it would be
+        # nice if we could, somehow.
+        rendered = self.schema.dump(event)
+        # FIXME: transmit rendered over the underlying channel
+        pass
+
+    def listen(self, callback: Callable[[Any], None], pattern: str = '#'):
+        def unwrapping_callback(message):
+            event = self.schema.load(message)
+            callback(event)
+        # FIXME: ask the underlying channel to call us back
+        # but only for messages matching pattern
+        pass
+
+    def __enter__(self):
+        self.connect()
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.close()
+
+
+# Predefined event channels for ease of use
+workflow_events = Channel(WorkflowEventSchema())