Add charm config option (or action) to provide private signing key

[Haw Loeung]

Hi,

At present, we manually manage this. This is a requirement within Canonical IS to prevent leaking the keys within Juju's controllers. Outside of Canonical IS, there needs to be a way to provide or even generate private signing keys. We should either expose it as a config option or as a Juju action.

With having it as a Juju action, it can generate, and another action to retrieve.

We'll want an option to either keep the signing keys consistent between multiple DKIM signing units or have separate signing keys for each (with a separate selector).