feat: add ros2 interface CLI command support

[furushchev]

Add Bazel macro and Python wrapper to enable ros2 interface command
functionality within Bazel-built ROS 2 projects.

Changes:

• Add interface.bzl macro for creating ros2_interface binary targets
• Add ros2_interface.py Python wrapper exposing interface CLI verbs
• Register new files in ros2/BUILD.bazel exports_files
• Add //chatter:interface as an example usage
  • This example will fail on test without ament.bzl: Use marker files instead of subdirectories in packages resource index #623

[mvukov]

A general question: why/how is this utility useful in Bazel context? -- I am just curious.

Naming is going to be tricky here. We already have interfaces.bzl -- without proper docs (new) folks might be puzzled with interface.bzl and interfaces.bzl. WDYT?

[mvukov]

Add yourself as a copyright holder in relevant files.

[furushchev]

A general question: why/how is this utility useful in Bazel context? -- I am just curious.

In my Bazel workspace, ROS2 interface definitions are converted from other sources and are not in workspace as files, so it's worth porting ros2 interface <command> tool into the workspace to ensure the conversion tool works correctly.

Naming is going to be tricky here. We already have interfaces.bzl -- without proper docs (new) folks might be puzzled with interface.bzl and interfaces.bzl. WDYT?

Currently, we have at least file level docstring both on interface.bzl and interfaces.bzl. Can you please clarify what you have in your mind about the proper docs in addition to them?

[mvukov]

A general question: why/how is this utility useful in Bazel context? -- I am just curious.

In my Bazel workspace, ROS2 interface definitions are converted from other sources and are not in workspace as files, so it's worth porting ros2 interface <command> tool into the workspace to ensure the conversion tool works correctly.

I don't get this. IIUC, this tool can be used to list interfaces used for a specific target. Is this correct? Also, what kind of conversion you have in mind?