Support custom Pathways args.

[RoshaniN]

Fixes / Features

• Adds support for adding Pathways proxy, server and worker args.
• Ensures indentation of added args is in line with the rest of the YAML.

Testing / Documentation

Manually verified -
Command -

python3 ~/xpk/xpk.py workload create-pathways \
--workload roshanin-pw-test${n} \
--num-slices=1 \
--tpu-type=$TPU_TYPE \
--project $PROJECT \
--zone $ZONE \
--cluster $CLUSTER \
--headless \
--custom-pathways-server-args='--temporary_flags_for_debugging=temporary_flag_for_debugging_worker_expected_tpu_chip_config=megachip_tccontrol'

YAML output -

template:
          spec:
            containers:
            - args:
              - --server_port=29001
              - --gcs_scratch_location=gs://cloud-pathways-staging/tmp
              - --node_type=resource_manager
              - --instance_count=1
              - --instance_type=tpuv6e:16x16
              - --temporary_flags_for_debugging=temporary_flag_for_debugging_worker_expected_tpu_chip_config=megachip_tccontrol          

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