feat: allow specifying API keys globally for site and instance

[Agrendalath]

Description

This implements the option to specify the API key for each model via:

1. Django admin (Site Configurations).
2. Django settings.

Testing instructions

1. If you are using Tutor, you can create the following YAML plugin to test this:

   name: dev_config
   version: 0.1.0
   patches:
     openedx-common-settings: |
       SITE_CACHE_TTL = 0  # To avoid cache delays after updating Site Configuration.
       XBLOCK_SETTINGS = {
          "ai_eval": {
              "GPT4O_API_KEY": "your-openai-api-key",
           }
       }

2. Use the instructions from the README to verify that the functionality works. If anything is missing, we should update them.

Other information

In the first two commits, I:

1. Dropped the Python 3.8 support, because it didn't support Self. I double-checked that the client does not use this version.
2. Converted existing tests to pytest to avoid mixing mark.parametrize from test_compat.py and ddt in the same repository. It's a general platform's direction to use pytest tests.

Private-ref: BB-9551

[ArturGaspar]

@Agrendalath Thank you for noting this!

[ArturGaspar]

@Agrendalath Looks great! Only one thing is that model_api_url should also be configurable by administrators.

[Agrendalath]

@ArturGaspar, sure thing - added in 7577830.

Edit: a quick heads-up that I had to rebase this PR to convert the unit test from 52fe3e4.

[ArturGaspar]

👍

• I tested this: Django settings are used when available, XBlock settings are used when configured
• I read through the code
• I checked for accessibility issues
• Includes documentation
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository.