[samples] DXOS Contacts sample app

[jessmartin]

Motivation

Writing an article to accompany the Effect Days conf talk on Schema and we need a sample app that's not Composer to show off how it works. We already have the hello app. This will be similar to that app, but also useful outside of conference contexts.

Finish Line

• Contacts app built on DXOS SDK
• Deploy to cloudflare
• Make sure README and GH repo is updated

[jessmartin]

Making progress! Contacts App

[jessmartin]

Need to add validations

[jessmartin]

Probably not going to add validations before Berlin - not necessary for the core talk I'm giving.