Added support for listing LDAP group links

[profhenry]

Fixes #1014

See https://docs.gitlab.com/ee/api/groups.html#list-ldap-group-links

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About the LdapGroupLink model:

The GitLab REST API documentation (https://docs.gitlab.com/ee/api/groups.html#list-ldap-group-links)
does not specify the attributes which are returned for a LDAP group link :-/.
This are the attributes returned by our GitLab instance (15.10.2-ee).
These seem fine because they match with the attributes which you can pass when adding a LDAP group link :-).

[jmini]

We should have an example JSON for LdapGroupLink.java tested like the other model class.

Otherwise the change looks good to me.

[jmini]

Looks good to me.

I hope the ldap-group-link.json I wrote is realistic enough (but I do not have a GitLab with LDAP to verify this)

[profhenry]

I think your ldap-group-link.json looks good.
Here is a group link returned by our gitlab (15.10.2-ee):

{
	"cn": "prj-rp-master-admins",
	"group_access": 50,
	"provider": "ldapmain",
	"filter": null
}

I think that one of cn or filter is always null (depending on how the group was added), but i think it is fine for a unit test.

[jmini]

merged!

@profhenry Thank you very much for your contribution and for the feedback here.