Add local pipeline development demo

@otto @tchin @lbowmaker @milimetric

Companion (demo) MR to eventutilities-python!3 (merged)

I added a demo of how to write local pipeline. It shows:

• loading mock data from a fixtures file
• feed it into a datastream, with schema validation
• running enrichment function on it
• collect results into a local sink, so they can be introspected and analysed.