feat: generate System Nexus interceptor points

[tconley1428]

Summary

• add the advanced-only --system-nexus generation option
• generate typed, operation-specific System Nexus workflow outbound interception points and their terminal dispatcher
• enable the option for the Python workflow-service example and check in its generated output

Companion Python SDK draft PR: temporalio/sdk-python#1791.

Validation

• cargo test --features advanced --test generate_python python_examples_generation_matches_checked_in_output -- --nocapture

[VegetarianOrc]

Overall looks okay! Couple minor questions/nits. Would also love to not have to thread the system_nexus bool a ton of places but not too particular about it and not worth refactoring a ton for.