add example low-level API for the chain head driver

[niklasad1]

Currently we have only provided one example how to initialize the chainHead backend with the background task provided by subxt, found here

However, it's also possible to use the low-level API where the user is responsible for polling the driver themselves and we need be extra clear that the driver to be run to completion to shutdown properly.

For instance if one bail on the first error seen, the stopped state may not be propagated properly. See #1817 for further explanation about that.