Expose rosidl buffer backend metadata in TopicEndpointInfo

[nvcyc]

Description

Expose buffer backend metadata through rclcpp::TopicEndpointInfo.

This lets C++ users of get_publishers_info_by_topic() and get_subscriptions_info_by_topic() access backend support metadata provided by the RMW graph.

Is this user-facing behavior change?

Yes. C++ graph introspection code can now read rosidl buffer backend support information from topic endpoint info.

Did you use Generative AI?

Yes. GPT-5.5 in Cursor was used to help draft changes in this pull request.

Additional Information

Depending on:

• Add rosidl buffer backend metadata to topic endpoint info rmw#419

[jmachowinski]

Can you give an example of what real world metadata would look like ?

[nvcyc]

Each vendor rosidl buffer backend can include a string that contains backend specific information/data (what we call as backend metadata) in the endpoint information that it wants to share with the same type of backends in other endpoints.

While the actual backend metadata really depends on each vendor's needs, a good example would be for a backend to share information like the GPU resources an endpoint has access to so the backends in other endpoints can determine if it's compatible or not.

[fujitatomoya]

lgtm with green CI.

[jmachowinski]

I got one point here, is this supposed to be human readable or is it just binary data ?

[jmachowinski]

My point would be, if it is binary, switch to

std::vector<std::byte>

instead of string.