🌱Add validation plugin and add tenant quota validation before creating a pod

[weiling61]

What this PR does / why we need it:

• Add validation plugin framework and interface
• Add tenant quota validation utilities
• Add validation plugin handling in pod syncer

Following is the proposal for this feature: https://github.com/kubernetes-sigs/cluster-api-provider-nested/blob/main/docs/proposals/20220206-quota.md

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Following issue was originally raised: Performance and Resource Contention in Multiple Tenant Environment #240

Fixes # Enhancement

[weiling61]

@Fei-Guo @christopherhein

[Fei-Guo]

Can we just have ValidationPluginInterface defined with three methods

type ValidationPluginInterface interface {
    Validation(client.Object, string) bool
    GetTenantLocker(string) *sync.Mutex
    Enabled() bool
}

so the changes to the Pod dws are straightforward. You can put rest of the code in resources/validationplugin/validationplugin.go to a separate file called quotavalaidationPlugin which implements the interface. I would suggest to name the plugin as QuotaValidationPlugin explicitly.

Also, please consider to move the validationplugin directory under resources/pod/ since the plugin works for Pod resource only.

For the plugin definition, can you just reuse the one that has been added in the util package? sigs.k8s.io/cluster-api-provider-nested/virtualcluster/pkg/util/plugin/plugin.go

[weiling61]

@Fei-Guo Thanks for the review. Updated according to your review comments

[weiling61]

@Fei-Guo updated according to your review comments.

[Fei-Guo]

What happened to the virtualcluster/README.MD file?

[Fei-Guo]

/lgtm /ok-to-test

[Fei-Guo]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Fei-Guo, weiling61

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