add config:clear subcommand

[benjodo]

https://app.shortcut.com/aptible/story/31724/add-config-clear-aptible-cli-command

[UserNotFound]

Any background on why this was requested or how it's intended to be used?

One gotcha would be that this implementation doesn't account for opeation concurrency:

• An operation is running which is adding a new value aptible config:set --app foo NEW=value
• Before that operation completes, someone runs aptible config:clear --app foo

The result is that the clears all but the NEW variable, because it wasn't none to the client at the time the operation was created, but will exist when it runs.

[benjodo]

Closing this PR for now, as there are valid concerns about how this feature would be used, and the impacts of its use and unintended consequences. config:unset remains a safer approach for our customers at this time. There are scripting workarounds to unset all variables as needed, such as:

APTIBLE_OUTPUT_FORMAT=json aptible config --app APP_HANDLE --env ENV_HANDLE | jq -r '.[].key' | xargs  aptible config:rm --app APP_HANDLE --env ENV_HANDLE

[UserNotFound]

Also, this deletes APTIBLE_DOCKER_IMAGE/APTIBLE_PRIVATE_REGISTRY_USERNAME/APTIBLE_PRIVATE_REGISTRY_PASSWORD which seems like a pretty big footgun.