[feature request] Support running colcon as a sub-process in a parallel build

[kunaltyagi]

Running colcon as top level process isn't always possible, and sometimes, colcon is run as part of a larger build (build server, pytest, etc.)

Is it possible for the parallel executor to conform to be a good jobserver interactive citizen? Example of existing implementation: https://pypi.org/project/pytest-jobserver/

Link to jobserver client requirements: https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html