Make server IO independently configurable in secnetperf

[mtfriesen]

Description

Describe the purpose of and changes within this Pull Request.

To independently test the client IO mode vs a constant server IO mode across IO models, add a new serverio knob.
Used by microsoft/netperf#629.

Testing

Do any existing tests cover this change? Are new tests needed?

Manually triggered run in netperf - https://github.com/microsoft/netperf/actions/runs/15030683055.

Documentation

Is there any documentation impact for this change?

No.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.78%. Comparing base (9da8ed5) to head (66a56a0).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5101      +/-   ##
==========================================
- Coverage   86.07%   85.78%   -0.29%     
==========================================
  Files          59       59              
  Lines       18035    18035              
==========================================
- Hits        15523    15471      -52     
- Misses       2512     2564      +52     

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