ucam_faas.gcp_pubsub provides support for parsing/validating/serialising
GCP Pub/Sub messages within CloudEvent envelopes. This is how
`functions_framework` exposes Pub/Sub messages to handlers when a
CloudRun function is subscribed to a Pub/Sub queue.

The module contains constants for the CloudEvent type that identifies
message events, and Pydantic data models for the data payload of these
events.

The decorator supports event handler functions that process message
events (currently GCP Pub/Sub messages, wrapped in CloudEvents). The
functions receive messages as instances of a data type they choose, that
represents the parsed payload of the message events they subscribe to.

This decorator allows event handler functions to accept message data as
a type representing the decoded payload of a message event. For example,
a Pydantic model representing a JSON payload.

- Validating incoming message metadata envelope
- Parsing/validating the message payload
- Recording the message envelope metadata in the function's
  `ExecutionInfo` result

Handler functions can focus on processing decoded message data payloads,
without needing to do their own parsing and validation or event metadata
logging/reporting.

This allows @message_handler(message_type=SomeModel) to parse a JSON
Pub/Sub message with the SomeModel model, and have the decorated handler
receive an instance of the model.

This allows @message_handler(message_type=MyMessage) to parse a Pub/Sub
message containing a Protobuf payload, where MyMessage is any Protobuf
Message class generated by the protoc compiler. The decorated handler
receives an instance of the message class.

The doctest example we had in the @message_handler docstring was
trying to be a representative example of a real handler function, but
this made it less easy to understand compared to a more minimal example.

We now present a basic "Hello World" type handler that does very little,
but should do enough to communicate the basics.

High-level handlers MR4: Message handlers

Closes #25, #27, #28, #29, and #30

See merge request !52