Allow customization of where RSA keys are stored

[namanshenoy]

Although setting RSA keys in the database makes things easy for the majority of users, it would be nice if we could customize where the RSA keys are retrieved from.
This enables us to use secret stores like AWS Secrets Manager, Vault, etc to store the private keys. Also enables the import, rotation, etc, of the keys from PKI providers.

I've created a PR #407 to enable this functionality via the use of a hook similar to the other hooks for this plugin.

Looking forward to a response! :)