Backchannel logout options about bff

[brockallen]

Done. Feel free to suggest a better name for the option.

        /// <summary>
        /// Specifies if during backchannel logout if all matching user sessions are logged out.
        /// If true, all sessions for the subject will be revoked. If false, just the specific 
        /// sesssion will be revoked.
        /// Defaults to false.
        /// </summary>
        public bool BackchannelLogoutAllUserSessions { get; set; }

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