[BB-9936] Make code execution tool configurable

[pkulkark]

Description

This PR adds a pluggable code execution backend for the Coding XBlock, allowing deployments to use either the default Judge0 service or a configurable third‑party service. The execution backend is setup via Django settings (AI_EVAL_CODE_EXECUTION_BACKEND). It also allows to optionally override the judge0 base URL via judge0_conf ig.base_url which can be used in case of self-hosted judge0 service. If no settings are provided, existing Judge0 behaviour continues unchanged.

Supporting information

OpenCraft's Internal Jira task: BB-9936

Testing instructions

1. Check out this branch and without changing any settings, verify that the default behaviour of using judge0 rapidapi works properly.
2. For a custom service, set the following django settings through tutor plugin:

AI_EVAL_CODE_EXECUTION_BACKEND = { 
    'backend': 'custom',
    'custom_config': { 
        'submit_endpoint': 'https://.../submit',
        'results_endpoint': 'https://.../results/{submission_id}',
        'languages_endpoint': 'https://.../languages',
        'api_key': '<token>',
        'auth_header_name': 'Authorization',
        'auth_scheme': 'Bearer',
        'timeout': 30
    }
}

3. Verify that the coding xblock works as expected.

Other information

Note: This change relies on the fact that the external service has an asynchronous model (similar to judge0) and separate submit_endpoint which returns a submission ID and results_endpoint which is polled with {submission_id}) for results. An additional languages_endpoint is used to validate supported languages.

[viadanna]

Approved 👍

• I tested this on my devstack.
• I read through the code.
• I checked for accessibility issues.
• Includes documentation.