Add support for configuring environment variables for review apps

[stof]

For apps, the config commands are allowing to manage the environment variables using the CLI instead of the dashboard. However, we are missing an equivalent reviewapps:config set of commands (the name is a suggestion) to configure the environment variables used when creating review apps in a pipeline. It would be great to have such commands.

[k80bowman]

@stof Thank you, I will pass this along to our product team.

[thatsanicehat]

Bumping this request. Having this feature would save us quite a bit of time.