Pathways in headless mode.

[RoshaniN]

Fixes / Features

• Enable Cloud DNS on Pathways clusters.
• Add user job conditionally based on whether headless mode is defined or not.
• Print out proxy address for Pathways in headless mode. Users would connect to this address from Vertex AI notebook / GCE VMs, to execute their JAX workloads.
• Organized XPK subcommands for Pathways.
• • Add cluster create-pathways subcommand. Deprecate --enable-pathways.
• • Add workload create-pathways subcommand. Deprecate --use-pathways.
• Update build tests and nightly tests, reordered a few tests.
• Restricting some features to create as they are not suitable for create-pathways, at the moment. (eg: autoprovisioning, debug dump to gcs, stack trace sidecar and restart on user failures.)

Testing / Documentation

Testing details.

• [ y ] Tests pass
• [ y ] Appropriate changes to documentation are included in the PR