Add custom validators, permission classes, and PermissionTestModel fo…

[Ghosts6]

This pull request introduces new custom validators, permission classes, and a test model to Django REST Framework:

• Added AlphabeticFieldValidator to validate alphabetic characters and spaces.
• Added AlphanumericFieldValidator to validate alphanumeric characters.
• Added CustomLengthValidator to enforce string length limits.
• Added IsAdminUserOrReadOnly permission class to allow only admin users to modify objects.
• Added IsOwner permission class to allow object owners to edit their own objects.
• Added PermissionTestModel for permission test cases.

All changes have been tested, and all tests pass.

[browniebroke]

According to our contributing page:

At this point in its lifespan we consider Django REST framework to be feature-complete. We focus on pull requests that track the continued development of Django versions, and generally do not accept new features or code formatting changes.

This is clearly a new feature, so no thank you.

PS: it would work nicely in a 3rd party package