Add rosidl buffer backend metadata to topic endpoint info

[nvcyc]

Description

Add rosidl buffer backend metadata to rmw_topic_endpoint_info_t so graph introspection APIs can carry backend support information for publishers and subscriptions.

Is this user-facing behavior change?

Yes. This enables higher-level ROS graph tools to expose buffer backend support information for topic endpoints.

Did you use Generative AI?

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

Additional Information

This PR only adds the RMW-level data model.
RMW implementations and client libraries need separate changes to populate and expose the metadata.

[Yadunund]

@nvcyc is there a test in ros2cli that we need to update as well?

See test failures across RMW impls: https://ci.ros2.org/job/ci_linux/29339/testReport/junit/ros2topic.ros2topic.test/test_cli/test_cli/

[fujitatomoya]

the struct previously held only const char * owning pointers, but it now holds a rcutils_string_map_t whose impl is a heap pointer with its own fini. any code path that copies an rmw_topic_endpoint_info_t by plain struct assignment or memcpy now aliases that impl between two structs.
the existing const char * members have the same hazard in principle, but the codebase already established a deep-copy discipline for strings.
i'd suggest a rmw_topic_endpoint_info deep-copy helper, or at minimum confirmation that no array/element copy in the current code and companion PRs.

[nvcyc]

Thanks for pointing this issue out.

I agreed that adding a field of type rcutils_string_map_t in rmw_topic_endpoint_info_t is exposing more possible failure surface and requires extra care when handling rmw_topic_endpoint_info_t.

For this reason, I'm updating rmw_topic_endpoint_info_t to host the newly added buffer_backend_metadata field with const char * type, which is consistent to what most other fields in rmw_topic_endpoint_info_t already use and hence not broadening the risk surface.

Corresponding serializer and parser functions are added to store and and get the rosidl buffer metadata.
Relevant PRs are also updated accordingly for this change.

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