Add the optional workload_id flag

[RUEI4341]

Description

Currently, the pw_mcjax_benchmark_recipe ignores user-specified workload names, instead defaulting to an auto-generated string composed of the username, model name, and a random 3-character suffix (e.g., roo-pw-default-1-ogo).

This PR introduces an optional --workload_id flag. When provided, the system bypasses the random name generation logic and uses the user's specific string for the XPK workload name and the MaxText run_name.,
This allows users to align XPK workloads with specific CI/CD jobs, internal experiment tracking IDs, and ensures predictable GCS output paths.

Tests

https://paste.googleplex.com/5792509692346368

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.