CONTRIB: Add a systemstap script to measure nested group code performance
    
    When all the dependencies are installed, run the script using systemtap:
    
    You'll see an output such as:
     # stap /usr/share/sssd/systemtap/nested_group_perf.stp
        ^CTime spent in group sssd_be searches: 600
        Time spent in sdap_nested_group_send/recv: 65 ms (ratio: 10.83%)
        Time spent in zero-level sysdb transactions: 6813 ms (ratio: 1135.50%)
    
        Breakdown of sdap_nested_group req (total: 65 ms)
            sdap_nested_group_process req: 64
                sdap_nested_group_process_split req: 22
                    sdap_nested_group_check_cache: 21
                        sdap_nested_group_sysdb_search_users: 10
                        sdap_nested_group_sysdb_search_groups: 9
                ldap request breakdown of total 60
                    sdap_nested_group_deref req: 24
                        sdap_deref_search_send req 24
                        processing deref results: 0
                    sdap_nested_group_lookup_user req: 18
                    sdap_nested_group_lookup_group req: 0
                    Time spent refreshing unknown members: 18
    
        Breakdown of results processing (total 6813)
            Time spent populating nested members: 10
                Time spent searching ldb while populating nested members: 5
            Time spent saving nested members: 110
            Time spent writing to the ldb: 678 ms
    
    Please note that since the script is supposed to be used in scenarios such as
    tracing "id" performance, which typically involve multiple group requests.
    Therefore, the variables are not zeroed out and you need to interrupt the
    script manually with Ctrl+C.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>