Enhancement: Queue rate counter support

[ssithaia-ebay]

What I did

• Added queue rate counters to queuestat (and therefore show queue counters):
• New JSON fields per queue: qpktsps, qbytesps, qbitsps.
• New table columns (appended at the end of all header variants): Pkts/s, Bytes/s, Bits/s.
• Kept default output shape backward-compatible (only appended columns; existing order unchanged).
• Added unit tests to validate headers, table rendering, and JSON output.

How I did it

• Extended queuestat headers: std_header, all_header, trim_header, and voq_header now end with ["Pkts/s", "Bytes/s", "Bits/s"].
• Included per-queue rate keys in the JSON path as numeric strings (qpktsps, qbytesps, qbitsps) to match existing field typing.
• Rate values are computed from counter deltas over time (first sample yields valid numeric strings; tests stub the data path to avoid DB deps).

How to verify it

pytest -q tests/queue_counter_test.py << all || trim || coq >>

This verifies:

• All headers end with Pkts/s, Bytes/s, Bits/s.
• json output contains qpktsps/qbytesps/qbitsps per queue (numeric strings).
• Default/trim/voq table paths render the new columns and show values.

Previous command output (if the output of a command-line utility has changed)

root@sonic:~# show queue counters
     Port    TxQ    Counter/pkts    Counter/bytes    Drop/pkts    Drop/bytes   
---------  -----  --------------  ---------------  -----------  ------------  

New command output (if the output of a command-line utility has changed)

root@sonic:~# show queue counters
     Port    TxQ    Counter/pkts    Counter/bytes    Drop/pkts    Drop/bytes    Pkts/s    Bytes/s     Bits/s
---------  -----  --------------  ---------------  -----------  ------------  --------  ----------  --------

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