Completion for dyno suzes should filter private dyno sizes

[stof]

Do you want to request a feature or report a bug?

Feature (nice-to-have)

What is the current behavior?

When providing completion choices for dyno sizes (for instance for --size of heroku run), all choices are shown, mixing private dynos and common runtime dynos

What is the expected behavior?

A better UX would be to display only sizes available for the runtime used by the app for which the command is running:

• for apps in private spaces, suggest only the Private-* dynos
• for apps in the common runtime, exclude Private-* dynos. And maybe also exclude Hobby dynos if the app is in an enterprise team.