[SDESK-7460] Create async auth rules for user privileges

[eos87]

Purpose

To implement privilege-based authentication rules for async rest endpoints.

What has changed

• Added new privilege rules and configurations to ensure that only users with the right permissions can access certain resources.
• Updated the test files to include scenarios for checking access to privileged resources.
• Introduced new utility functions to help with privilege and role management. Also another one to flat nested lists.
• Fixed json serialisation issue when saving session (SecureCookieSessionInterface not being able to handle ObjectId values).
• Made some small tweaks to the error handling and configuration files to support the new privilege system.
• Added tests to make sure the new rules work as expected.

Pending

- I will push some changes to fix all (unrelated) mypy issues tomorrow. Done!

Resolves: SDESK-7460

Thanks for checking!