Add ability to override jwks_url

[KapilSareen]

Fixes #8348

Proposed Changes

• Make jwks-uri configurable, allowing the default value to be overwritten by passing a value in JWKSURI in the feature config.
• Default to an empty string (""), and use the JWKS URI provided by oidc-discovery-base-url if no jwks-uri is specified.

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature
• Conformance test for any change to the spec

[knative-prow]

Welcome @KapilSareen! It looks like this is your first PR to knative/eventing 🎉

[knative-prow]

Hi @KapilSareen. Thanks for your PR.

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: KapilSareen
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[pierDipi]

/cc @creydr

[maschmid]

We should probably have at least an oidc- prefix, so, maybe oidc-jwks-uri ?