Resolve ldap-password with Castellan

This feature allows you to pass the ldap-password option as a vault reference so the ldap-password won’t be in plaintext in the config files.

When ldap-password is passed as a vault reference, the charm will create the necessary config changes in the domain config file and render additional castellan config files so that castellan can resolve the ldap-password using the vault backend at runtime.

In order to get the vault credentials, a new secrets-storage relation was added.