[DESIGN] Should ALL peer data handled as secret?

In order to elliminate the potential for developer error (i.e. "forgetting to register a peer relation field as a secret field"), should we potentially consider a specific interface function that makes sure that a field is added as a secret?

I.e. what could be called from the charm's `set_secret()` method, ensuring that the field will be considered as a secret
So it would dynamically add the field to `secret-fields` if needed (see https://github.com/canonical/data-platform-libs/issues/127, and add the value specified.

This should be very simple to do, by literally calling the new function to be added, let's say `update_secret_fields(new_field)`, and `update_relation_data()`.