Add new decorator to pass a received GCP PubSub message received object to a handler function

Description

Add new decorator to the library to validate and parse a PubSub MessagePublishedData event and return the data in the message.data to the handler function as its original binary.

Further details

This decorator should live under a pubsub_messages module, and be called message_handler (or check current name in the draft MR and use that).

The handler should have access to the lower level CloudEvent and MessagePublishedData objects - see original MR.

Task list

Acceptance criteria

• Added decorator to extract data from a MessagePublishedData payload and pass as binary to the handler function
• On errors these should be logged and a 400 response sent to caller

Links/references