Add support for updating env variables without restarting app

At present, the `heroku config:set` and `:unset` CLI commands will restart the app immediately after applying the env variable change.

This might not always be desirable. For instance, if I'm adding env variables that will be only consumed by the next release of the app, it does not make sense to restart the currently deployed app.
Coming from Cloud Foundry I've found this very surprising, as `cf` client doesn't do implicit restarts after manipulating environment but rather has a dedicated `restage` command for that purpose.

Please consider adding an option to the `heroku config:set` and `:unset` CLI commands that would prevent restarting the app.

My environment is:

```
$ heroku --version
heroku/7.42.5 linux-x64 node-v12.16.2
```