tools/cephfs_mirror: Add read/write throughput

tools/cephfs_mirror: Add read/write throughput

The read throughput added measures the bytes
read per second from the source ceph filesystem.
Similarly, the write throughput added measures
the bytes written per second to the remote ceph
filesystem. It's derived from the time spent
in preadv and pwritev calls.

Sample output:
-------------
{
    "/d0": {
        "state": "syncing",
        "current_syncing_snap": {
            "id": 2,
            "name": "d0_snap0",
            "sync-mode": "full",
            "avg_read_throughput_bytes": "12.69 MiB/s",
            "avg_write_throughput_bytes": "54.49 MiB/s",
            "crawl": {
                "state": "completed",
                "duration": "1s"
            },
            "bytes": {
                "sync_bytes": "149.94 MiB",
                "total_bytes": "149.94 MiB",
                "sync_percent": "100.00%"
            },
            "files": {
                "sync_files": 5000,
                "total_files": 5000,
                "sync_percent": "100.00%"
            }
        },
        "snaps_synced": 0,
        "snaps_deleted": 0,
        "snaps_renamed": 0
    }
}
-------------

Fixes: https://tracker.ceph.com/issues/73453
Signed-off-by: Kotresh HR <khiremat@redhat.com>