Add opt-in support for Kubernetes user namespace isolation on sandbox
pods. When enabled, container UID 0 maps to an unprivileged host UID
and capabilities become namespaced, providing defense-in-depth for the
supervisor process.
Configuration is two-layered: a cluster-wide default via
OPENSHELL_ENABLE_USER_NAMESPACES (default false) and a per-sandbox
override via the new `user_namespaces` field on SandboxTemplate.
When user namespaces are active, the pod security context is extended
with SETUID, SETGID, and DAC_READ_SEARCH capabilities to match the
bounding-set requirements inside a user namespace.
Introduces SandboxPodParams struct to replace long argument lists on
sandbox_to_k8s_spec and sandbox_template_to_k8s.
Validated end-to-end on OCP 4.22 (K8s 1.35.3, CRI-O 1.35, RHEL
CoreOS, kernel 5.14) with full SSH tunnel and non-identity UID mapping.
