Support HashiCorp Vault AppRole authentication

Allow to pass role_id and secret_id instead of token

When using AppRole authentication actual tokens can be retrieved by providing the role and secret IDs, but the returned token is then only valid for a limited amount of time; so the plugin would need to be able to retrieve a new token every once in a while.

Right now it is not possible to use MariaDB Server with such a Vault setup as it is not even possible to set a new token at runtime, only at startup. So the server would have to be restarted every 20-30 minutes ...

Exposing hashicorp-key-management-token as a write-only variable to replace the current token might work as an easier to implement workaround for the time being, maybe?

See also:

https://developer.hashicorp.com/vault/docs/auth/approle