Allow EventSource/Sensor defaults to be set in a ConfigMap

[JacobHenner]

Is your feature request related to a problem? Please describe.

In order to comply with enforced pod security standards, each pod created by an EventSource or Sensor must be configured with specific SecurityContext values (defined in .spec.template.securityContext in a Sensor or EventSource). This is also true for other fields in .spec.template - they must be set on each Sensor/EventSource.

Right now, there is no built-in mechanism available to set default values. Cluster operators might want to impose default .spec.template fields for various reasons.

Describe the solution you'd like

Instead of requiring all users to include boilerplate configuration in each Sensor/EventSource, operators should be able to specify defaults in a ConfigMap. A similar feature exists today in Argo Workflows.

Describe alternatives you've considered

Cluster operators could also implement a mutating admission webhook to set Sensor/EventSource .spec.template defaults, but a built-in defaults feature seems more appropriate and user-friendly.

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