Add "update all" command to update all created projects

Scenario:
1. I have created a macOS and an iOS project.
1. I make a code change that I am happy with using `briefcase dev`
1. I want to push the code to all created projects with one command, it has been suggested that `briefcase update all` might fit the current standards
* briefcase in the background would determine what types of projects have been created (if it doesn't already have that data) and run the update commands for those projects