[config] add `exportUrl` field

[esamelson]

follow-up from https://github.com/expo/expo-cli/pull/2275#discussion_r441900987

Would be nice to have an exportUrl field (or similar) in app.json which would accomplish two purposes: (1) keep user from having to enter the same --public-url every time they run expo export and (2) let us set the update URL field to the correct value in expo eject.

[github-actions[bot]]

This issue is stale because it has been open for 60 days with no activity. If there is no activity in the next 7 days, the issue will be closed.

[EvanBacon]

npx expo export no longer asks for a public URL