[Chore] Minor Improvements

[stevensJourney]

Overview

This PR adds some minor improvements which make running the service locally easier.

• Added VSCode dev container config for developing with Python and Postgres in a isolated dev environement - this could be removed if not deemed helpful. I found it helpful when developing on this repo.
• Added a Dockerfile config for using this in a Docker compose project.
• Moved Postgres publication creation to a Django migration
• Added the ability to use a default key pair for JWT and JWK functions
• Added a registration endpoint for creating users.
• Made listen port configurable and exposed service to all hosts.

Related PRs

• This work is used in the self hosting demo [Feature] Demos self-host-demo#17
• The React Native Demo is updated Django RN demo app: fix some issues picked up during Django backend cleanup powersync-js#217