access_users: Add ZeroTrust Users & Seats functions.

[gavinelder]

Description

Closes #1425

The following adds the Cloudflare Zerotrust / Access endpoints for user & seat management.

The following is a rough draft with comments for feedback.

Has your change been tested?

Basic API calls locally against relevant endpoints to ensure a proper response is given.

Types of changes

What sort of change does your code introduce/modify?

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas
  and relies on stable APIs.

[github-actions]

changelog detected ✅

[gavinelder]

Feel free to add relevant comments .

Right now as far as testing goes, outside of basic response unmarshelling I am unsure what you would like to see.

[gavinelder]

@jacobbednarz thanks for the review, this should hopefully be in a better state now.

[jacobbednarz]

thanks for this one @gavinelder 🏆

[github-actions]

This functionality has been released in v0.81.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!