Add support for ignoreApplicationDifferences resource customization key for ApplicationSet resources

[sbathgate]

Summary

It would be very helpful to support an ignoreApplicationDifferences customization key for ApplicationSet resources in the same way other ignore logic (e.g. ignoreDifferences, ignoreResourceUpdates) works. This would allow users to ignore specific fields such as /spec/syncPolicy or .spec.sources[].targetRevision when comparing live vs. desired states in Argo CD.

Motivation

When using ApplicationSet, there are certain fields (e.g., syncPolicy or targetRevision fields) that can get frequently updated or differ from the desired specification in ways the user wants to ignore. Currently, we can configure various ignore behaviors via "ignoreDifferences" or "ignoreResourceUpdates," but there's no dedicated mechanism for ignoring differences on ApplicationSet fields specifically.

For example, in the Argo CD ConfigMap (argocd-cm), we cannot specify:

resource.customizations.ignoreApplicationDifferences.argoproj.io_ApplicationSet:
  - jsonPointers:
      - /spec/syncPolicy
  - jqPathExpressions:
      - .spec.sources[].targetRevision

However, Argo CD does not yet recognize a key named resource.customizations.ignoreApplicationDifferences.<group_kind> to process these ignore configurations properly for ApplicationSet resources.

Proposal

Enhance the appendResourceOverridesFromSplitKeys (and related logic) to handle a new customization type ignoreApplicationDifferences. This would likely involve:

1. Introducing a new case block in the function (similar to ignoreDifferences, ignoreResourceUpdates) that processes the ignoreApplicationDifferences key.
2. Parsing the YAML from the ConfigMap data into a structure (e.g., v1alpha1.OverrideIgnoreDiff) and storing it on a new or existing field in ResourceOverride.
3. Updating any necessary docs or comments to clarify usage of ignoreApplicationDifferences.