Add `xacro` support

[frneer]

It would be good to have a xacro rule that allows generating URDF/SDF artifacts.

Given the following xacro

<!-- urdf/robot.urdf.xacro -->
<robot xmlns:xacro="http://www.ros.org/wiki/xacro">
  <!-- Should be able to compose with local `.xacros` -->
  <xacro:include filename="$(find my_robot)/urdf/macros.xacro"/>
  <!-- Should be able to bring `.xacros` installed in the system (via `realsense2_description` for this example)-->
  <xacro:include filename="$(find realsense2_description)/urdf/_d435i.urdf.xacro"/>
   <!-- Should be able to retrieve xacro args-->
   <xacro:arg name="sim" default="true" />
</robot>

Example API

# BUILD.bazel
load("@bazel_ros2_rules//lib:ament_index.bzl", "ament_index_share_files")
load("@bazel_ros2_rules//lib:xacro.bzl", "ros_xacro")

# Registers custom ROS package "my_robot" with its share files.
ament_index_share_files(
    name = "my_robot_index",
    package_name = "my_robot",
    srcs = glob(["urdf/**"]),
)

ros_xacro(
    name = "robot.urdf",
    src = "urdf/robot.urdf.xacro",
    ros_packages = [
        ":my_robot_index",                      # custom package via ament_index_share_files
        "@ros2//:realsense2_description_share", # system package share files
    ],
    xacro_args = ["sim:=true"],
)

[jwnimmer-tri]

For reference, both Drake and Anzu have an implementation already available that might serve as a starting point:

• drake/tools/workspace/ros_xacro_internal/defs.bzl
• anzu/models/tools/xacro.bzl

[julianadrianheine]

Thanks @frneer for the description of what is needed. This work will be tackled here #422