Feature Request: Adding an option "Group member attribute" to LDAP Authentication Servers, to allow use of servers which dont use memberOf

[jakobsen-lrz]

Important notices

Before you add a new report, we ask you kindly to acknowledge the following:

• [x ] I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• [x ] I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.
Our LDAP Servers dont use the memberOf attribute to specify group memberships. It uses a custom Attribute for this purpose
We are currently looking in to migrating to opnsense, but this would make the migration quite a bit more difficult, as the workaround would involve having a Authserver for each group, and specifying that group as the default group for each individual server.

Describe the solution you like

I would like an option "Group member attribute", in the config of LDAP Authentication Servers, that allows specifying the LDAP Attribute to check for group memberships.

Describe alternatives you considered

I cant think about a different way to implement it that would work in our case.

Additional context

I have implemented this option myself and it seems to be working fine on my Test-Firewall. So i would like to open a PR to have it merged into OPNsense.

[jerryhze]

I have a related issue here.
#9286

If a work around is presented in this post, then I can work around my issue as well, so just cross posting here for reference.