Fix issue #1679: Add deprecation of interface variables

[Pengkun-ZHU]

Companion to ros2/rosidl#946 (fixes ros2/ros2#1679)

Summary

Adds DeprecationWarning support to the Python message generator for fields annotated with @deprecated.

What this changes

rosidl_generator_py/resource/_msg.py.em

For any field whose IDL Member carries a @deprecated annotation (set via # @deprecated in the .msg source), the generated Python property getter and setter now emit:

import warnings
warnings.warn(
    "field 'old_field' in 'MyMessage' is deprecated",
    DeprecationWarning,
    stacklevel=2)

This fires at runtime whenever user code reads or writes the deprecated field, following standard Python deprecation convention (stacklevel=2 points the warning at the caller, not the generated code).

Coverage

The _msg.py.em template is the single leaf template invoked for all message-like types — messages, service request/response/event sub-messages, and action goal/result/feedback sub-messages — so the warning propagates automatically to .srv and .action fields as well.

Known limitations / out of scope

• Whole-message deprecation is not addressed here; only field-level.
• No custom deprecation message — # @deprecated carries no free-text argument in this version.
• Wire format is unchanged — @deprecated does not affect type hash or serialization.

[InvincibleRMC]

I can write my own implementation or you can update yours. Either is fine but you would have to add the deprecation push pop probably to the message.c.em. You should also use deprecation from typing_extensions. And finally use the text from the annotation some where in the message so user can know what to migrate to

[Pengkun-ZHU]

Sure, I will make a further change to align with your implementation.