Support user namespaces in Serving

## Describe the feature

Kubernetes v1.25 will add support for [user namespaces](https://github.com/kubernetes/enhancements/tree/master/keps/sig-node/127-user-namespaces) as an alpha feature. This will improve security through increased pod to node/pod isolation and should also help with the multi-tenancy work stream.

For now, I'm proposing doing some initial exploratory work around what it would take to use user namespaces for Knative pods.

/area API
/area security

/assign

Start with the Kubernetes user namespaces KEP linked in the issue, then inspect the Knative Serving code paths that configure pods. Determine what exploratory support for user namespaces would require and document the findings, constraints, and next steps. The issue does not name specific files or tests.

Written by the indexing model from the issue text.