Allow unsetting all secrets at once

[nhedger]

Hey :wave:

This is a feature request.

---

I'd like to be able to unset all the secrets of an application at once.

# No secrets names unsets all secrets
flyctl secrets unset

# Alternatively, using a --all flag
flyctl secrets unset --all

Either implementation could optionally prompt the user for confirmation in interactive sessions.

I'd be willing to contribute the feature, but just wanted to check if that's something you'd be open to.

Workaround

Currently, I'm using the following workaround:

flyctl secrets unset $(flyctl secrets list --json | jq -r '[.[].Name] | join(" ")')