Bump to 0.2.0

[esteve]

This PR updates version number to 0.2.0 in preparation for a new release.

[codecov-commenter]

Codecov Report

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #30   +/-   ##
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  Coverage        ?   62.11%           
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  Files           ?        8           
  Lines           ?      388           
  Branches        ?       50           
=======================================
  Hits            ?      241           
  Misses          ?      138           
  Partials        ?        9           

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[luca-della-vedova]

Should we put this on hold until we know how to distribute cargo-ament-build on packagecloud?

[esteve]

@luca-della-vedova I think it's safe to merge this PR as it'll streamline the installation process for the extension and the cargo plugin now that the plugin is available via PyPI (i.,e. we can just pip install the extension and the plugin will be installed as well)

[luca-della-vedova]

Imho it would be great to get #31 in before this, so we can have a graceful failure where we tell the user what to do if cargo-ament-build is not installed and we can release this and colcon-cargo on packagecloud