feat: ldap cluster connector

[phiremande]

Overview

This changes implements a new ldapcluster connector, which helps configure multiple ldap servers and requests are tried in round robin fashion to these servers with an affinity to last active server.

What this PR does / why we need it

#1548

Special notes for your reviewer

Does this PR introduce a user-facing change?

Introduces a new connector 'ldapcluster', which helps configure a cluster of ldap servers.  This can be used to provide fault tolerant LDAP connection.

[scottd018]

+1 to this MR. Have an enterprise customer that wishes to use Dex but has concerns with lack of redundancy provided by the connector.

[sagikazarmark]

Thanks for the contribution. TBH, I don't think this is the best way to tackle the problem. I think it should be handled on the connection/LDAP connection level.

I asked about an existing solution in the LDAP client repo. Let's see what they answer.

[stefanmcshane]

Hi All,

We are discussing expanding the reach of the ldap library, and adding support for clustering connections.

Any ideas or usecases you have would be appreciated on go-ldap/ldap#314

Thanks,
Stefan