refactor(controllers): extract nested endpoints into separate classes

[theEvilReaper]

Proposed changes

Some controller implementations have endpoints which handle nested data objects from their root entity. Currently, each nested endpoint is located in the root controller, and this results in unreadable implementations. This pull request overhauls the controller structure to make it more readable and maintainable.

Types of changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of
them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before
merging your code.

• I have read the CONTRIBUTING.md
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[theEvilReaper]

I tested the changes locally and everything works as expected