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

[CChemin]

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.