Add @key annotation

[SuperJappie08]

Description

Add parsing for @key in rosidl_adapter.
This enables the use of keyed topics with msg, srv and action files.

Fixes #939

Is this user-facing behavior change?

Yes, users can now use keyed topics with msg, srv and action interface files instead of idl.

Did you use Generative AI?

No

Additional Information

[asymingt]

Thanks for the contribution :) We've marked this PR for discussion in next week's ROS PMC meeting. We should be able to get back to you within the week.

[asymingt]

This was discussed in the PMC meeting on 2026-03-24. It was pointed out that support for this keyed topics Zenoh is unknown, plus CycloneDDS for example doesn't appear to support keyed topics. Members wanted to ensure that we do our best not to leak DDS-specific features into the ROS domain, and if we choose to do so that we understand the consequence of our decision to do so! That being said, they weren't opposed to this PR, but they do want to put out a REP to solicit feedback from the wider community. Is this something you'd be willing to lead?

[SuperJappie08]

@asymingt, sorry for the delayed response.

I understand the concerns from the PMC.

When I made this PR I was not aware of the implications it would have to the ROS communication setup.
I just noticed that @optional was supported in the rosidl conversion, while @key was not, which felt off.

I agree creating a REP for this makes sense, however I don't think I have enough understanding of the RMW layer(s) and DDS mechanisms to be able to lead this effort.