:sparkles: [ENH] Allow to have secret variables at a realm level

We want to be able to fetch and write a realm with secret variables. For example to override `fr.insee.sugoi.ldap.default.password` so that each store provider can have its own password.

Of course we cannot store this secret unencrypted.

It is possible to implement a solution using a service designed to store secrets (such as hashicorp vault) but I think we should also implement a more straightforward method.

Start by locating the realm fetch and write entry points and how realm variables are currently stored. Determine the encryption and key-management approach for realm-level secret variables, including overriding provider passwords, then document the behavior and tests needed to verify encrypted storage and retrieval.

Written by the indexing model from the issue text.