feat(rabbit): support EXTERNAL authentication

[WellFREEzZ]

Description

Add first-class RabbitMQ EXTERNAL authentication support.

This introduces RabbitExternalAuth, allowing users to authenticate to RabbitMQ with TLS client certificates and the auth=EXTERNAL AMQP URL parameter without mutating broker internals. The change also documents the new security mode, adds a runnable docs example, covers AsyncAPI security output, and fixes Windows-specific mypy issues in CLI supervisor signal handling.

Fixes: N/A

Type of change

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• This change requires a documentation update

Checklist

• My code adheres to the style guidelines of this project (just lint shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running just test-coverage
• I have ensured that static analysis tests are passing by running just static-analysis
• I have included code examples to illustrate the modifications

Verification

• just linter passed
• just test-rabbit tests/brokers/rabbit passed: 188 passed, 1 xfailed
• Targeted Rabbit/AsyncAPI/docs tests passed: 20 passed
• PYTHONUTF8=1 PYTHONIOENCODING=utf-8 just static-analysis passed
• just zizmor passed
• git diff --check origin/main..HEAD passed

just test-coverage currently fails locally on tests/cli/test_asyncapi_docs.py::test_serve_asyncapi_docs_from_app_with_reload. The same test fails on current main in the same environment, so it does not appear to be caused by this change.

[CLAassistant]

All committers have signed the CLA.