feat(bench): add --username/--password and reuse admin client

[hubcio]

Authentication was hardcoded to default root credentials and
post-benchmark operations created throwaway clients. Propagate
-u/-p credentials through ClientFactory, reuse a single admin
client in the runner. Reassign -p from --messages-per-batch
(now -P) to --password for iggy CLI compatibility.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.26%. Comparing base (ce5c739) to head (d3551e4).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2778      +/-   ##
============================================
- Coverage     68.28%   68.26%   -0.03%     
  Complexity      656      656              
============================================
  Files           741      741              
  Lines         62165    62165              
  Branches      58578    58578              
============================================
- Hits          42450    42436      -14     
- Misses        17605    17619      +14     
  Partials       2110     2110              

Flag	Coverage Δ
rust	69.32% <ø> (-0.03%)	⬇️

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see 13 files with indirect coverage changes

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