[Feature] Demos

[stevensJourney]

Overview

This PR adds a Django demo alongside the current NodeJS demo. Both demos are Docker compose projects which share common service defintions.

Readme files have been updated with instructions.

Docker compose health checks have been added to speedup the Mongo replica set initialization and guard Django Postgres migrations. In some cases the PowerSync instance would crash if MongoDB timed out (when doing migrations), health checks now also sequence the starting of PowerSync to avoid this.

Testing

Verified NodeJS demo functions correctly:

nodejs_demo.mov

Tested Django demo using React Native Django demo: powersync-ja/powersync-js#217

Todos

• Update Django demo after merging changes