Add acceptable_buffer_backends field in SubscriptionOptionsBase

[nvcyc]

Description

This pull request adds the acceptable_buffer_backends subscription option in rclcpp, which corresponds to the subscription field with the same name introduced in rmw in ros2/rmw#416. This option allows subscribers to control which buffer backends they accept, with a backward-compatible default of CPU-only.

This pull request consists of the following key changes:

• rclcpp (subscription_options.hpp): SubscriptionOptionsBase adds acceptable_buffer_backends (std::string, default "cpu"). to_rcl_subscription_options() copies it into rmw_subscription_options.acceptable_buffer_backends when non-empty.
• Generic subscription (generic_subscription.hpp): Constructor forces acceptable_buffer_backends to "cpu" ensuring serialized messages received by generic subscriptions are always CPU-based, which will be used by ros2 bag record for direct storage in bag files.

Is this user-facing behavior change?

The default value "cpu" preserves existing behavior -- subscriptions only accept CPU-backed data unless explicitly opting in. This is a new API surface but is additive and backward-compatible. Existing code that does not pass acceptable_buffer_backends will behave identically to before.

Did you use Generative AI?

No.

Additional Information

This PR is part of the broader ROS 2 native buffer feature introduced in this post.
It depends on the subscription field with the same name introduced in rmw in ros2/rmw#416.

[hidmic]

First pass

[hidmic]

LGTM

[nvcyc]

Since all subscription is set to only accept "cpu"-based messages by default, the forced opts.acceptable_buffer_backends = "cpu"; setting in the generic subscription is becoming redundant, and hence removed with the commit 67f9def

[nvcyc]

Pulls: #3098
Gist: https://gist.githubusercontent.com/nvcyc/8016e85eb31d063db11f8e4596250258/raw/14dc7eafd4837dd461d8e8c891538169d60b06f0/ros2.repos
BUILD args: --continue-on-error --packages-above-and-dependencies rclcpp
TEST args: --packages-above rclcpp
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/18752

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[nvcyc]

Pulls: #3098
Gist: https://gist.githubusercontent.com/nvcyc/0b931d69c44789ba3f6b9ee7543467fb/raw/14dc7eafd4837dd461d8e8c891538169d60b06f0/ros2.repos
BUILD args: --continue-on-error --packages-above-and-dependencies rclcpp
TEST args: --packages-above rclcpp
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/18772

• Linux
• Linux-aarch64
• Linux-rhel
• Windows