Add OpenLDAP support

[derbl4ck]

It would be awesome if OpenLDAP support is added to the Repo. There are already other projects out there (e.g. https://github.com/Nordes/IdentityServer4.LdapExtension or https://github.com/jusbuc2k/Justin.AspNetCore.LdapAuthentication) which did some work on that topic.

Example config settings

    "BindDn": "cn=adminuser,dc=damienbod,dc=com",
    "BindCredentials": "P@ss1W0Rd!",
    "SearchBase": "ou=users,DC=damienbod,dc=com",
    "SearchFilter": "(&(objectClass=person)(cn={0}))"

Advantages

• The Project would serve an IdP with MFA support and LDAP Integration
• Usage of existing UserStores via LDAP
• Usage of On-Premises OpenLDAP instead of AzureAD