Add testing_uri as output of the step

[theeuwes-it]

I'm using the "Post to Slack" step in my workflow, and would like to post the testing uri from firebase to Slack so the testers can download the new app. Firebase CLI returns three values when you distribute to Firebase (firebase_console_uri, testing_uri and binary_download_uri). It would be nice to add those to the outputs of the step.

Reference: https://firebase.google.com/docs/app-distribution/android/distribute-cli

[guness]

this is a good feature. I should add these on the next release.