Add FeatureGate: ConfigurablePKI

[sanchezl]

Add ConfigurablePKI feature gate.

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[openshift-ci]

Hello @sanchezl! Some important instructions when contributing to openshift/api:
API design plays an important part in the user experience of OpenShift and as such API PRs are subject to a high level of scrutiny to ensure they follow our best practices. If you haven't already done so, please review the OpenShift API Conventions and ensure that your proposed changes are compliant. Following these conventions will help expedite the api review process for your PR.

[openshift-ci]

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