Add mixin to colcon-common-extentions

[sloretz]

Relates to #18

I notice python3-colcon-mixin isn't installed with ros-dev-tools, and isn't part of python3-colcon-common-extensions.

Thoughts on adding it? The mixin documentation is already included in the "reference" section of the colcon documentation, so it would seem natural to me to make it one of the default plugins.

https://colcon.readthedocs.io/en/released/reference/verb/mixin.html

[Ryanf55]

Relates to colcon/colcon-mixin-repository#36

I agree, this is a great enhancement. If you follow the ROS tutorials, then follow the colcon docs, there's a gap.

[Ryanf55]

@mjcarrol While it's on your mind from the other PR, do you have any thoughts on this addition? If you like it, I'm happy to do a PR.