Allow sharing of cargo target directory between builds

[cottsay]

This flag will cause all colcon-cargo invocations of the 'cargo' executable to use a shared target directory, thereby sharing build artifacts between builds.

I don't yet understand all of the nuances of this change and what dragons be yet lurking, so for now the change is hidden behind the colcon feature flag 'shared_cargo_target_dir'.

To avoid breaking the _build_cmd and _install_cmd signatures with colcon-ros-cargo, I saved the build directory as an attribute. We may need to untangle that relationship in the future.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 18.18182% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.63%. Comparing base (518a4f9) to head (5ae3fa3).

Files with missing lines	Patch %	Lines
colcon_cargo/task/cargo/build.py	18.18%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
- Coverage   72.46%   71.63%   -0.84%     
==========================================
  Files           9        9              
  Lines         345      349       +4     
  Branches       60       62       +2     
==========================================
  Hits          250      250              
- Misses         61       64       +3     
- Partials       34       35       +1     

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[Blast545]

LGTM since it's a feature flag, the nuances are not clear to me either, but I'm ok with the opt-in behavior.

[esteve]

@cottsay doesn't this break the assumption that package builds are isolated? e.g. if we build them in a chroot a la Debian. Not actually against it, but it'd be good to document it.