Completion for dyno suzes should filter private dyno sizes

Do you want to request a *feature* or report a *bug*?
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Feature (nice-to-have)

What is the current behavior?
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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?
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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.