Explore local build management

The steps for releasing a new version of an app are:

1. Export archive
2. Export build from archive
3. Zip build
4. Notarize zip and wait
5. Stop notarization to build
6. Zip the stapled build
7. Copy new zip to releases dir
8. Render changeling from file
9. Run generate_appcast
10. Upload files to server

Right now these are done through a combination of shell scripts and UpdateServer. I think it would be worth combining these into a single build pipeline that can be configured for each project. It would also be a good way to test writing a Package.swift-style config tool, rather than JSON.