Add webhook for EventPolicy

[creydr]

We need to add a webhook for EventPolicies, with at least the following validations for EventPolicies:

• An EventPolicy must not be created, when the authentication-oidc feature flag is disabled
• EventPolicySpecTo must only contain either a Ref or a Selector. Not both at the same time

Prerequisites:

• Add EventPolicy type #7971
• Add default-authorization-mode feature flag #7974

Additional context:

• Feature track document

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2. Make sure you've read and understood the CONTRIBUTING.md guidelines
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[pierDipi]

EventPolicySpecTo must only contain either a Ref or a Selector. Not both at the same time

I did (perhaps wrongly) include this validation logic in this PR: #7995

[dharmjit]

Hi @creydr @pierDipi , If okay, I can take this up.

[creydr]

Hey @dharmjit,
would be great, if you could work on this. The other required issue (#7974) should be done soon the the PR merge (I just lgtm'ed it).

/assign @dharmjit

[creydr]

Hey @dharmjit,
as the prerequisite issues are resolved, this should be ready to be worked on (moved from "Backlog" to "Ready").

[rahulii]

Hey @dharmjit , let me know if you don't have sufficient bandwidth so that I can take this up while you are working on the reconciler part (which itself is a big part)
Thanks!

[Leo6Leo]

@dharmjit are there anything I can help you with?

If you're still working on this issue, please let me know within the next 24 hours. We understand that plans and priorities can change, and if you're no longer able to continue with this task, that's completely okay! In case I don't hear back from you in the next 24 hours, I'll reassign the issue to @rahulii to ensure our workflow continues smoothly.

[dharmjit]

@Leo6Leo Apologies, I got caught up in some stuff. I will try to pick this up tomorrow and submit a PR.