Drop the edx-token-utils dependency

[feanil]

Context

Currently the edx-platform repo depends on the edx-token-utils package, that package lives in the edx org and not in the openedx org. From what I can tell, the edx-platform only uses one function which looks like it's a thin wrapper around pyjwkest.

• edx-token-utils is a library created to handle non-auth-related JWTs, which edx-exams introduced to validate exam-related JWTs.
• This PR is where Zach introduced edx-token-utils into edx-platform for use with edx-exams: feat: gate exam content using access token #31653.

Given the simplicity of the functions and the lack of use anywhere else, I think it makes the most sense to just fold in the necessary parts into edx-platform and drop the dependency.

Acceptance Criteria

• edx-platform has no dependencies on edx-token-utils

Implementation Notes

This library is also referenced in the edx-exams repo maintained by 2U in the edx github org. If the code is folded in here, then it would be good to have a comment to indicate the specification it's following and where to find that specification.