Add publish implementation for Google Play Store #2698
Description
What is the problem or limitation you are having?
Briefcase has a publish command; however, it is only a stub implementation for Android Gradle targets.
Google provides the Google Play Developer API to manage the submission and publication process. It should be possible to use this API to automate submitting an Android app to the Play Store.
Describe the solution you'd like
briefcase publish android -c playstore should publish an app to the Google Play store.
Describe alternatives you've considered
Continue to require manual Play Store submission, potentially removing the publish command.
Additional context
This would be a good follow on from #2687.
Ideally, the submission process would also include creating the full Play Store submission, including descriptions, screenshots etc. It's not clear how much of this can be automated, and how much will always require a user to log in and fill in details, but if there's content being provided, the first goal should be to be able to provide that content locally, stored in CI.
It's also not clear if the process will be different for first submission and subsequent submissions.
Implementing this also requires a fix for #1268.
The Google Developer API has Python bindings.
Gradle Play Publisher is an unofficial Gradle plugin to automate submission.
Fastlane is a tool that provides a pre-wrapped user experience; it could be considered a competitor to briefcase publish. It could also be considered as an implementation mechanism - however, it is a Ruby-based tool, and only works on macOS, so it might not be viable for Android projects.