Adds a PermissionService sample for Python

[markmcd]

This is pretty basic, but I find myself coming back to it, especially for sharing tuned models with service accounts. Adding it to the repo in case it's helpful for others too.

Type of change

Choose one: Other

Checklist

• [x] I have performed a self-review of my code.
• [x] I have added detailed comments to my code where applicable.
• [x] I have verified that my change does not break existing code.
• [x] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [x] I am familiar with the Google Style Guide for the language I have coded in.
• [x] I have read through the Contributing Guide and signed the Contributor License Agreement.