This function creates default_factory functions that source values from
`ContextVar`s. Pydantic models and dataclasses can use these default
factory functions to automatically set properties from their
environment.

This decorator takes care of the common functionality that typical
event-handler functions need when they process CloudEvents. It provides
support for:

- Validating the type of incoming CloudEvents
- Parsing/validating the data payload of CloudEvents
- Recording the CloudEvent's metadata in the function's ExecutionInfo
  result

Handler functions can focus on processing a primary data value without
needing to do their own parsing and validation or event metadata
logging/reporting.

To use this module, handler functions are wrapped with this module's
`@cloud_event_handler(...)` decorator. The decorator takes a parser
function that is responsible for validating CloudEvent values, and
extracting their data payload.

Handler functions receive the primary value from the parser function,
and also have access to the the full CloudEvent via the
`CLOUD_EVENT_HANDLER_CONTEXT` `ContextVar`. Metadata from the CloudEvent
is automatically included in `CloudEventHandlerExecutionInfo` values
created within the handler function's scope.

Contextual event handler functions receive a primary value as an
argument, with additional context value provided to implicitly using a
`ContextVar`.

This separation allows a handler function to act primarily on an
important subset of incoming event data, but still access the wider
event metadata when needed, and automatically include metadata in
`ExecutionInfo` result values (using the `contextual_default_factory()`
pattern).

Incoming events are separated into the primary value and context value
by a parser function that receives the incoming event value from the
parent handler.

The `@contextual_handler(...)` decorator is the interface to this
functionality.

The doctest example we had in the @cloud_event_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 MR2: Contextual handlers

See merge request !50

High-level handlers MR3: CloudEvent handlers

Closes #26

See merge request !51