docs: add ADR for standardizing permissions usage

[taimoor-ahmed-1]

Description

Currently, authorization logic is implemented inconsistently across views, serializers, and custom access checks. This ADR will define a consistent approach using DRF permission classes, object-level permissions, and queryset scoping where appropriate.

Issue: #38177

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[rodmgwgu]

My initial thoughts:

Although I'm not very familiar with DRF permission classes, this seems like a good approach, I just want to bring some points to the table:

My team is currently working on the RBAC AuthZ project, which aims to improve roles and permissions handling across Open edX, see: https://openedx.atlassian.net/wiki/spaces/OEPM/pages/5364121605/PRD+AuthZ+for+Course+Authoring

We are currently implementing the new permissions system over the CMS, specifically on Course Authoring features.

For now, we are focusing on direct replacements on old permission checks, we recently merged a decorator that is a direct replacement for a legacy one, which supports validating a permission either on the new system or the old one depending on a feature flag: #38156

I think we could probably implement the same thing with this approach (I'll do some reading on it).

However, there are some more involved cases, like the xblock endpoint being implemented here: #38179, where more granular permissions mean that different checks need to be done depending on the request params, I wonder if DRF permission classes could handle this, or we would need to refactor the endpoint (may not be feasible at least for this specific endpoint), or doing an exception.

Thanks for the proposal, I'll do some reading to get more familiar with DRF permissions so we can start using it in our project.

FYI @wgu-taylor-payne @dwong2708