[YUNIKORN-1338] [admission] support different kind of workloads other than pods

[pbacsko]

What is this PR for?

Handle non-pod workloads in the admission controller.

To have a better design & readability, usergroup.go and usergroup_test.go has been moved to a new package. Also, related functions are now bound to a type - they are not global anymore in the "main" package.

What type of PR is it?

• [ ] - Bug Fix
• [x] - Improvement
• [ ] - Feature
• [ ] - Documentation
• [ ] - Hot Fix
• [ ] - Refactoring

Todos

• [ ] - Task

What is the Jira issue?

https://issues.apache.org/jira/browse/YUNIKORN-1338

How should this be tested?

Screenshots (if appropriate)

Questions:

• [ ] - The licenses files need update.
• [ ] - There is breaking changes for older versions.
• [ ] - It needs documentation.

[codecov[bot]]

Codecov Report

Merging #467 (327877c) into master (8ff45c3) will increase coverage by 0.26%. The diff coverage is 95.91%.

:exclamation: Current head 327877c differs from pull request most recent head 3859fa3. Consider uploading reports for the commit 3859fa3 to get more accurate results

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
+ Coverage   68.03%   68.30%   +0.26%     
==========================================
  Files          42       42              
  Lines        6921     6989      +68     
==========================================
+ Hits         4709     4774      +65     
- Misses       2042     2044       +2     
- Partials      170      171       +1     

Impacted Files	Coverage Δ
pkg/plugin/admissioncontrollers/webhook/webhook.go	40.00% <88.88%> (ø)
...missioncontrollers/webhook/admission_controller.go	70.78% <91.07%> (+1.01%)	:arrow_up:
...missioncontrollers/webhook/annotation/usergroup.go	100.00% <100.00%> (ø)
pkg/cache/amprotocol_mock.go	54.02% <0.00%> (-0.53%)	:arrow_down:
pkg/cache/application.go	71.78% <0.00%> (-0.18%)	:arrow_down:
pkg/cache/context.go	44.96% <0.00%> (-0.08%)	:arrow_down:

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