Validate and use filters in the flow order

[samutamm]

• Fix issue described in Failed to create pipeline when filters are not sorted as flow #326
• minor refactoring to reuse code in httppipeline.Spec.Validate() and httppipeline.HTTPPipeline.reload()

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 348 Deploy Test Success

[codecov-commenter]

Codecov Report

Merging #348 (44b28fc) into main (5a95804) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #348      +/-   ##
==========================================
+ Coverage   80.48%   80.58%   +0.09%     
==========================================
  Files          54       54              
  Lines        6217     6217              
==========================================
+ Hits         5004     5010       +6     
+ Misses        947      942       -5     
+ Partials      266      265       -1     

Impacted Files	Coverage Δ
pkg/cluster/cluster.go	49.82% <0.00%> (+1.05%)	⬆️

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[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 348 Deploy Test Success

[megaeasex]

[TASK:easegress-pr-test FAILED]megaease/easegress Pull Request 348 Deploy Test failed

[megaeasex]

[TASK:easegress-pr-test FAILED]megaease/easegress Pull Request 348 Deploy Test failed

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 348 Deploy Test Success

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 348 Deploy Test Success