Add xacro support to bazel_ros2_rules

[julianadrianheine]

Description

This PR introduces a macro ros_xacro that allows generating URDF/SDF artifacts. The implementation was based on #420

Summary

• Adds bazel_ros2_rules/lib/private/xacro.bzl with a ros_xacro macro that transforms a .urdf.xacro source file into a .urdf output file at build time
• Registers xacro.bzl in COMMON_FILES_MANIFEST so it is distributed as @ros2//:xacro.bzl
• Adds ros2_example_xacro as a worked example and test for the new rule

Implementation notes

ros_xacro follows the same pattern as ros_launch: it wraps a generated runner script in ros_py_binary, whose dload shim sets up AMENT_PREFIX_PATH (including prefixes for any user-defined packages) before executing the system @ros2//:xacro binary. Resolved runfiles paths are always absolute, which is required because xacro's $(find <pkg>) expansion joins non-absolute paths with the parent file's directory.

Local packages are declared inline via the ros_packages dict, mapping a ROS package name to its source files. The macro creates an ament_index_share_files target for each entry internally, deriving strip_prefix automatically from native.package_name() so the user never has to compute it.

The BUILD file usage is minimal:

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

ros_xacro(
    name = "example",
    src = "example.urdf.xacro",
    ros_packages = {
        "my_robot": glob(["urdf/**"]),
    },
    data = [macros.xacro],
    xacro_args = ["sim:=false"],
)

---

This change is 

[frneer]

+a:@frneer

@frneer partially reviewed 9 files and made 1 comment.
Reviewable status: 8 of 12 files reviewed, all discussions resolved.

[frneer]

@frneer made 1 comment.
Reviewable status: 8 of 12 files reviewed, all discussions resolved.