feat(appset): implement ListResourceEvents for ApplicationSet

[alexymantha]

Part of the effort to add support for ApplicationSets in the UI, adds a ListResourceEvents endpoint to list the events on an Application, similar to the ListResourcesEvents on the Application. I kept the ListResourcesEvents name for consistency, even though the only resource is the ApplicationSet in this case.

Closes #15753

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• [x] Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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• [x] I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them. (Part of another PR)
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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 49.26%. Comparing base (f87897c) to head (2018f46). Report is 18 commits behind head on master.

:exclamation: Current head 2018f46 differs from pull request most recent head 6292aeb. Consider uploading reports for the commit 6292aeb to get more accurate results

Files	Patch %	Lines
server/applicationset/applicationset.go	66.66%	6 Missing and 3 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15754      +/-   ##
==========================================
- Coverage   49.73%   49.26%   -0.48%     
==========================================
  Files         274      274              
  Lines       48948    48197     -751     
==========================================
- Hits        24343    23742     -601     
+ Misses      22230    22105     -125     
+ Partials     2375     2350      -25     

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[reggie-k]

@alexymantha I have merged your code into mine to test end to end - and now the new API on the backend side gets invoked without any changes in the client side! There are no events for my AppSet, how can I make events get created so that I can test how it looks?

[alexymantha]

@reggie-k That's great! For the events, I did my testing by manually creating them for the AppSet using something like this:

 kubectl apply -f - <<EOF
apiVersion: v1
kind: Event
metadata:
  name: test-appset-event
  namespace: argocd
type: Warning
message: 'This is a test event'
reason: 'TestingEvents'
involvedObject:
  kind: ApplicationSet
  name: <NAME>
  namespace: argocd
  uid: <UID>
EOF

You can also try to make operations on ArgoCD to trigger an event, but I found it was easier to just create them manually for testing.

[reggie-k]

Thank you @alexymantha for the event manifest. @alexymantha @crenshaw-dev Manual end to end tests on this PR, using the UI, passed, for both returning no events and for returning the actual events for the AppSet. If no comments from other perspectives and if no additional end to end testing required, this PR can be merged.