fix(appimage): bundle dlopen'd RMW and typesupport libraries

[facontidavide]

Fixes #10.

Summary

ldd only reports link-time dependencies. The RMW providers and the rosidl_typesupport_* libraries are loaded with dlopen, so they were never copied into the AppDir and every 0.9.0 AppImage aborted on a clean machine with failed to load any RMW implementations.

• Bundling moved from inline YAML into packaging/make_appdir.sh, so it can be run and tested locally instead of only in CI.
• Copies both RMW providers and every typesupport flavour, then walks transitive dependencies from the original files — a copy's $ORIGIN rpath no longer resolves, which silently truncated the closure.
• No RMW is forced. RMW_IMPLEMENTATION is left to the environment (it has to match the user's other nodes); both providers are shipped so either choice works.
• Bundled message definitions remain a fallback: AMENT_PREFIX_PATH is appended, so a sourced workspace keeps priority. The AppImage is not self-sufficient for custom message types, by design.

Size goes from ~20 MB to ~20 MB packed (AppDir 49 MB → 69 MB).

Test plan

• Built a real AppImage locally from the Humble pixi env
• Runs in a clean ubuntu:24.04 container with no ROS installed — the exact repro from AppImage (all distros) missing RMW provider .so — dlopen'd libs not captured by ldd bundling #10
• End-to-end: host publishers, bridge in the container, client decodes 3 topics at 20 Hz
• RMW_IMPLEMENTATION unset and rmw_cyclonedds_cpp both work (matching publishers)
• Type missing from the bundle: skipped with a clear error without ROS sourced, resolves with a workspace sourced
• CI builds all three AppImages

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