chore(example): migrate fastapi provider example

This migrates the Pact FastAPI provider from the old standalone
examples, and merges it with the new combined examples.

The consumer tests are executed first which publishes the contracts with
the broker. The provider test is then executed against the broker to
verify compliance with the published contracts.

This does, at this stage, create an unusual interdependence between
tests which typically should be avoided. I plan to fix this at a later
stage.

Signed-off-by: JP-Ellis <josh@jpellis.me>

Author: JP-Ellis