Add Iron support (based on iron/20240327)

[gavanderhoorn]

This builds, but needs (more) testing with MotoROS2.

Requires corresponding changes to _buildscripts.

NOTE: this does not (yet) track upstream HEAD of relevant Iron-related branches. It's slightly older.

Main outstanding issue: access to remapping argument parsing functionality has changed significantly in upstream micro-ROS (so much as to break our build). See Yaskawa-Global/motoros2#52 for the discussion.

Opening as draft, as we'd probably want to update this to track upstream more / use a more recent Iron (patch) release.

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Edit: builds of MotoROS2 will need Yaskawa-Global/motoros2@main...gavanderhoorn:iron_no_remap_rules.

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Edit 2: test build: micro_ros_motoplus_yrc1000-iron-20240327-rc0_1719833791.zip

[gavanderhoorn]

Note: this does not include #2 (need new patches for Iron).

[jimmy-mcelwain]

@gavanderhoorn I have a local commit with the updated upstream versions of these branches for iron. Would you like me to push that?

[gavanderhoorn]

Perhaps a PR against iron_update_20240327? So we can review the updates independently?

[ted-miller]

[iron_update_20240327](https://github.com/Yaskawa-Global/micro_ros_motoplus/tree/iron_update_20240327) appears to be working well. It passes all our tests.

I assume that we should just close this PR and rename iron_update_20240327 to iron

[gavanderhoorn]

I assume that we should just close this PR and rename iron_update_20240327 to iron

I would actually suggest to branch off a new iron branch from current HEAD of humble, update the target of this PR to be that new iron branch and then we could merge it.