(Draft) ApplicationSet Progressive Sync

[wmgroot]

-- THIS IS STILL A DRAFT --
Functionality is working, but unit tests still need to be added.
Please feel free to comment.

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Checklist:

• 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.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
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Example ApplicationSet spec used for E2E testing.

---
apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: hello-world-nginx
spec:
  generators:
  - clusters:
  ...
  syncPolicy:
    preserveResourcesOnDeletion: true
  strategy:
    type: RollingUpdate
    rollingUpdate:
      steps:
        - matchExpressions:
            - key: rolloutWave
              operator: In
              values:
                - tier-5
          maxUpdate: 0 # if undefined or 0, all applications matched are updated together
        - matchExpressions:
            - key: rolloutWave
              operator: In
              values:
                - tier-3
          maxUpdate: 1 # maxUpdate supports both integer and percentage string values
  template:
    metadata:
      name: 'hello-world-nginx-{{name}}'
      annotations:
        indeed.com/argo-cd-instance: test-mgroot
        indeed.com/applicationset: hello-world-nginx
      labels:
        rolloutWave: "{{criticality}}"  # label can be provided explicitly from a list generator or from cluster secret labels, etc
    spec:
      project: hello-world-nginx
      source:
        repoURL: <internal repo with a simple dummy nginx Deployment>
        path: charts/nginx
        targetRevision: master
        helm:
          releaseName: hello-world-nginx
          values: |-
            clusterName: {{name}}
            deployment:
              template:
                annotations:
                  test: "30"   # this field is incremented to test the progressive rollout capability, nginx pods become ready after 2 mins
      destination:
        name: '{{name}}'
        namespace: hello-world-nginx--demo
      syncPolicy:
        automated:
          prune: true
          selfHeal: true
        syncOptions:
          - ApplyOutOfSyncOnly=true
          - CreateNamespace=true

Example ApplicationSet status progression.

# App #1 and App #2 stable on same AppSet version
status:
  applicationStatus:
  - application: hello-world-nginx-awscmhdev
    lastTransitionTime: "2022-07-19T22:17:29Z"
    message: Application resource becaame Healthy, updating status from Progressing
      to Healthy.
    observedGeneration: 3
    status: Healthy
  - application: hello-world-nginx-awscmhqa2
    lastTransitionTime: "2022-07-19T22:17:29Z"
    message: Application resource becaame Healthy, updating status from Progressing
      to Healthy.
    observedGeneration: 3
    status: Healthy


# App #1 Progressing, App #2 Waiting
status:
  applicationStatus:
  - application: hello-world-nginx-awscmhdev
    lastTransitionTime: "2022-07-19T22:56:58Z"
    message: Application resource became Progressing, updating status from Pending
      to Progressing.
    observedGeneration: 4
    status: Progressing
  - application: hello-world-nginx-awscmhqa2
    lastTransitionTime: "2022-07-19T22:56:58Z"
    message: Application is out of date with the current AppSet generation, setting
      status to Waiting.
    observedGeneration: 3
    status: Waiting

# App #1 Healthy, App #2 Progressing
status:
  applicationStatus:
  - application: hello-world-nginx-awscmhdev
    lastTransitionTime: "2022-07-19T22:59:04Z"
    message: Application resource becaame Healthy, updating status from Progressing
      to Healthy.
    observedGeneration: 4
    status: Healthy
  - application: hello-world-nginx-awscmhqa2
    lastTransitionTime: "2022-07-19T22:59:06Z"
    message: Application resource became Progressing, updating status from Pending
      to Progressing.
    observedGeneration: 4
    status: Progressing

# App #1 and App #2 on new AppSet version
status:
  applicationStatus:
  - application: hello-world-nginx-awscmhdev
    lastTransitionTime: "2022-07-19T22:59:04Z"
    message: Application resource becaame Healthy, updating status from Progressing
      to Healthy.
    observedGeneration: 4
    status: Healthy
  - application: hello-world-nginx-awscmhqa2
    lastTransitionTime: "2022-07-19T23:01:12Z"
    message: Application resource becaame Healthy, updating status from Progressing
      to Healthy.
    observedGeneration: 4
    status: Healthy

[wmgroot]

Closing in favor of the official PR here.
argoproj#10048