Enable ARM64 build for rmw_connext

[lobolanja]

Description

Enable ARM64 (Ubuntu) builds to achieve official ARM support for rmw_connext. To accomplish this, changes will be required in at least the following repositories.
https://github.com/ros2/ci
https://github.com/ros/rosdistro
https://github.com/ros2/rmw_connextdds

Motivation

Update the CI to enable building rmw_connextdds for ARM64 on Ubuntu. Since we previously provided ARM packages through rti-connext-dds-7.3.0-ros, enabling this build would allow users to install the Connext RMW implementation directly instead of building it from source when running on ARM machines.