Add support for mypy plugins to enable django-stubs testing in typeshed library

[huynguyengl99]

We can support Django type checking via mypy's django-stubs plugin. The required mypy.ini configuration is simply:

[mypy]
plugins = mypy_django_plugin.main

[mypy.plugins.django-stubs]
django_settings_module = scripts.tests_settings

With a minimal tests_settings.py file:

SECRET_KEY = "1"
INSTALLED_APPS = (
    "django.contrib.contenttypes",
    "django.contrib.sites",
    "django.contrib.sessions",
    "django.contrib.messages",
    "django.contrib.admin.apps.SimpleAdminConfig",
    "django.contrib.staticfiles",
    "django.contrib.auth",
    "channels",
)

I propose we extend our typing tools to support mypy plugins by adding the "plugins" field to the KNOWN_METADATA_TOOL_FIELDS dictionary:

KNOWN_METADATA_TOOL_FIELDS: Final = {
    "stubtest": {
        "skip",
        "apt_dependencies",
        "brew_dependencies",
        "choco_dependencies",
        "extras",
        "ignore_missing_stub",
        "platforms",
        "stubtest_requirements",
    },
    "mypy": {
        "plugins",
    }
}

This would allow package maintainers to specify in their METADATA.toml:

[tool.mypy]
plugins = "mypy_django_plugin.main"

[tool.mypy.plugins.django-stubs]
django_settings_module = "scripts.tests_settings"

This enhancement would enable proper stubtest functionality for Django-based stubs libraries. What do you think about this idea?

[sobolevn]

I will be happy to help from django-stubs side :)