chore: use Header arguments

What does this merge request do and why?

Currently, because we strictly check the header in our endpoint, the "example" request from the docs will always be 400, which is annoying.

This MR adds optional Header parameters to the user JWT method, so that it shows up in the FastAPI UI:

How to set up and validate locally

Start AI GW, go to the docs page and check the code/user_access_token method

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Closes #500 (closed)