Expo Router Express Middleware

[tyrauber]

This PR updates the example to SDK 50 and implements the manifest and assets api endpoints in an expo-router express server using express middleware. The benefit of this approach is that the middleware can be packaged as a library and used independently of the example. Tests are included within the package.

This PR has been linted, tested, and run on simulators.

[a-eid]

I think your PR is missing the code-signing-keys directory on the server.

[tyrauber]

@wschurman, What do you think about replacing nextjs with expo-router for the custom-expo-updates-server example?

[wschurman]

@tyrauber - this repo is intended to be a very small, basic demonstration of the expo-updates protocol in real code, so I think we'll keep it as-is. That being said, feel free to fork it!