Add Active Directory group filter

[jch]

This adds an additional filter to support AD when querying for groups. We can add an integration test for this later on, but I'll include results from manual testing in a bit.

cc @mtodd

[mtodd]

👍 Would love to see a comment with a specific mention of Active Directory.

[mtodd]

Aside from the comment, this should be sufficient to fix the issue.

@jch adding a test for Domain#filter_groups would be really great! Especially if we can validate it against Active Directory with our local test VM.

[jch]

I was able to get the following test working in test/domain_test.rb, but it breaks the other tests in the file b/c of how we skip integration tests. I'm going to file a separate issue for cleaning up AD testing and adding the test from there.

class GitHubLdapActiveDirectoryGroupsTest < GitHub::Ldap::Test
  def run(*)
    self.class.test_env != "activedirectory" ? super : self
  end

  def test_filter_groups
    domain = @ldap.domain("DC=ad,DC=ghe,DC=local")
    results = domain.filter_groups("ghe-admins")
    assert_equal 1, results.size
  end
end

@mtodd cool to merge this as-is, and cutting a release from here? Alternatively, I can create another test file with just this test in it and merge them back together in another file.

[mtodd]

@jch let's cut the release and then worry about adding tests.