Fixes: #7755

Description

Currently, third-party addons must be passed via CLI flags (-s /path/to/addon.py), loaded via configuration, or imported in custom wrapper scripts.
This PR introduces automatic discovery and loading of addons registered under the entry point group mitmproxy.addons.

This enables packaging third-party addons into standalone Python packages (e.g. on PyPI) that are automatically activated when installed into the same environment or virtualenv as mitmproxy.

Key changes:

1. Entry Point Discovery (mitmproxy.addons.entrypoint_addons):
   • Queries importlib.metadata.entry_points(group="mitmproxy.addons").
   • Supports classes (automatically instantiated), zero-argument factories/functions (invoked), pre-instantiated addon objects, or sequences of addons returned by a factory.
   • Automatically assigns the entry point name as addon.name if the loaded addon does not already specify one.
   • Catches and logs errors gracefully with logger.error if a broken or incompatible third-party addon fails to import or instantiate, ensuring mitmproxy starts reliably regardless of third-party issues.
2. Integrated into default_addons():
   • Discovered entry point addons are appended at the end of default addons, preserving existing core precedence.
3. Tests:
   • Added exhaustive unit tests in test/mitmproxy/addons/test_entrypoints.py covering all discovery patterns, error resilience, and integration with default_addons().
   • Verified 100% individual coverage with uv run tox -e individual_coverage.
4. Changelog:
   • Updated CHANGELOG.md under ## Unreleased: mitmproxy next.

How to use

In pyproject.toml of an addon package:

[project.entry-points."mitmproxy.addons"]
my_addon = "my_package.addon:MyAddon"

Once installed into the environment (pip install my_package / uv pip install my_package), mitmproxy will automatically discover and load my_addon.

Checklist

• Implemented entry point discovery in mitmproxy.addons.
• Unit tests added and passing (uv run pytest test/mitmproxy/addons/test_entrypoints.py).
• 100% test coverage verified (uv run tox -e individual_coverage -- test/mitmproxy/addons/test_entrypoints.py).
• Linting and code formatting verified (uv run ruff check . and uv run ruff format --check .).
• Type checking passed (uv run mypy).
• CHANGELOG.md updated.