Add simple async_context example

[mjcross]

A simple example showing how to add an at-time worker to a background async context.

The intention is to supplement the High Level API documentation in the SDK which doesn't really seem to go into the practical details of how to create and add workers.

The example builds and runs for both Pico and Pico-W boards so I suggest it belongs as a top level example rather than in the Pico-W folder. I've put it in an async_context folder on the assumption there might in due course be other examples (e.g. when-pending worker).