Provide a list API of all steps

[JohnNiang]

What this PR dose

Provide a list API of all steps.

Please see: https://github.com/jenkinsci/blueocean-plugin/tree/master/blueocean-rest#get-pipeline-steps

[codecov-commenter]

Codecov Report

Merging #29 (ea6c5df) into main (d28f47e) will increase coverage by 0.10%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##             main      #29      +/-   ##
==========================================
+ Coverage   69.36%   69.46%   +0.10%     
==========================================
  Files          35       35              
  Lines        1890     1903      +13     
==========================================
+ Hits         1311     1322      +11     
- Misses        513      514       +1     
- Partials       66       67       +1     

Flag	Coverage Δ
unittests	69.46% <84.61%> (+0.10%)	⬆️

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

Impacted Files	Coverage Δ
pkg/job/blueocean.go	95.74% <84.61%> (-1.79%)	⬇️

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[JohnNiang]

/cc @LinuxSuRen