feat(coder/openshift): add OpenShift workspace template

[Edd88-pixel]

Description

Why

Creating an OpenShift workspace currently requires manually adapting the Kubernetes template and selecting a compatible image. This adds a dedicated template that works with the default restricted-v2 SCC.

Changes

• Add registry/coder/templates/openshift, based on the existing Kubernetes template.
• Let OpenShift assign the workspace UID and fsGroup, without requiring additional SCC permissions.
• Use a digest-pinned UBI-based developer image that supports arbitrary UIDs, preserving its entrypoint.
• Persist /home/user in a PVC across workspace stops and restarts.
• Document authentication, project setup, provisioner permissions, storage requirements, and SCC behavior.
• Add Terraform tests, including independent coverage of default and explicit StorageClass selection, and an OpenShift icon.

Type of Change

• New template

Template Information

Path: registry/coder/templates/openshift

Tested on a real OpenShift 4.21.30 cluster with Coder 2.36.7, AMD64 nodes, and the gp3 StorageClass.

The end-to-end checks confirmed agent connectivity under restricted-v2, terminal and code-server access, file persistence across stop/start, and cleanup of the workspace Deployment, pod, and PVC on deletion. The tested OpenShift version is documented in the README.

Related Issues

Closes #1162