Add Keep-Alive flag for Zipkin HTTP server

[topjung3]

Which problem is this PR solving?

• Resolves [Feature]: Allow configuring KeepAlive for Zipkin collector #4365

Short description of the changes

• Add cli flag for change jaeger-collector's Zipkin HTTP server KeepAlive configuration.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 ⚠️

Comparison is base (499145f) 97.08% compared to head (42a28db) 97.07%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4366      +/-   ##
==========================================
- Coverage   97.08%   97.07%   -0.02%     
==========================================
  Files         300      300              
  Lines       17697    17701       +4     
==========================================
+ Hits        17182    17183       +1     
- Misses        414      416       +2     
- Partials      101      102       +1     

Flag	Coverage Δ
unittests	97.07% <100.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/collector/app/collector.go	88.00% <100.00%> (+0.09%)	⬆️
cmd/collector/app/flags/flags.go	100.00% <100.00%> (ø)
cmd/collector/app/server/zipkin.go	92.72% <100.00%> (+0.13%)	⬆️

... and 1 file with indirect coverage changes

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