BE: RBAC: LDAP: Implement `user` subject type for LDAP & AD

[Haarolean]

          Can we implement support for `user` subject type like here https://github.com/kafbat/kafka-ui/pull/719/files#diff-8a3d6c3f4c752c32a92b1b123bf39c98c4d6ee1377ad1ee1636999bdc9410a87R39 ?

Or will we do it later as a separate PR?

Originally posted by @wernerdv in #717 (comment)

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TODO: Implement a user subject type for LDAP/AD RBAC:

        - provider: ldap
          type: usee
          value: "haarolean"