Add support for fieldSelector to fakeClient

[grzesuav]

Currently it seems only labelSelector is supported - https://github.com/kubernetes-sigs/controller-runtime/blob/master/pkg/client/fake/client.go#L423

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/lifecycle stale

[matteoolivi]

If there's consensus from the project maintainers me and @pleic would like to open a PR to add this, since we have code that needs it. If there's not immediate consensus (e.g. because a real API server should be used with envtest rather than the fake client) we'll elaborate more on our use case and try to make a case for this.

Our rough idea to implement this was to:

1. add a WithIndex(gvk schema.GroupVersionKind, IndexName, IndexFunc) function to the fake client builder.
2. add a field of type map[schema.GroupVersionResource]map[string]IndexFunc that stores all the Indexes that have been registered with the fake client on a per GroupVersionResource basis.
3. when a List requests specifies a field selector, we intercept the normal list results and filter it by lazily computing all the relevant indexes and dropping the list items that don't satisfy the selector.

cc @alvaroaleman @vincepri since you reviewed the previous attempt at this: #800

[alvaroaleman]

sounds good to me

[FillZpp]

/remove-lifecycle stale

[matteoolivi]

PR opened: #2025

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all issues and PRs.

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/lifecycle stale

[alvaroaleman]

Fixed in #2025