Enhancement request: Add support to search within multiple organisational units in ldap

**Is your enhancement request related to a problem? Please describe.**

We have a complex structure in our Active Directory, which means that not all our users can be found under a single organisational unit. Currently, the implementation of django-auth-ldap in Dejacode only supports searching within a single organisational unit, which would mean that not all users are found and therefore cannot log in to Dejacode and use the system.

https://dejacode.readthedocs.io/en/latest/application-settings.html#user-dn

**What are the benefits of the requested enhancement?**

As it is a common ‘problem’ that the structure within an Active Directory in companies becomes large and complex, I am probably not the only one encountering this.

The extension offers greater flexibility when using LDAP integration in large organisations.

**Describe the solution you would like**

I would like to see an implementation of https://django-auth-ldap.readthedocs.io/en/stable/authentication.html#search-unions. https://github.com/aboutcode-org/dejacode/blob/d12d2d0dfd4e5a9ee8a6db4c7491f8c9fa61f715/dejacode/settings.py#L770