[JENKINS-64830] Add AGGREGATION_ONLY option in UI

[uhafner]

While the steps support this new type the UI and help still lists only the old types. For details see JENKINS-64830.

[codecov]

Codecov Report

Merging #800 (743c39b) into master (7922938) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #800      +/-   ##
============================================
+ Coverage     80.03%   80.14%   +0.10%     
- Complexity     1559     1560       +1     
============================================
  Files           243      243              
  Lines          5659     5660       +1     
  Branches        418      418              
============================================
+ Hits           4529     4536       +7     
+ Misses          974      968       -6     
  Partials        156      156              

Impacted Files	Coverage Δ	Complexity Δ
...ns/plugins/analysis/core/util/ModelValidation.java	69.31% <100.00%> (+7.24%)	37.00 <0.00> (+1.00)

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