add support for additional build-types, e.g., ros2 pkg create --build-type anewbuildtype

This enhancement request is to enable the creation of packages for build-types other than c/c++ and python, i.e.,ros2 pkg create --build-type mynewbuildtype

Specifically I'm proposing the addition of an extension point for additional build-types. Something like this:

entry_points={
        'ros2cli.extension_point': [
            'ros2pkg.build_type = ros2pkg.build_type:BuildTypeExtension',
        ],

As an experiment I implemented this extension point and it was quite simple to work into ros2pkg.verb.command.py with minimal changes, no refactoring of existing build-types required. I've been using my implementation for creating javascript/rclnodejs projects. I would be glad to share a PR for your consideration. If included in ros2cli, we (the rclnodejs team) will release a new build type for creating nodejs projects that properly work with ros2 run and ros2 launch commands.

Thoughts?

Start in ros2pkg.verb.command.py and review the proposed ros2cli.extension_point entry for build types. Check how the existing C/C++ and Python build types are selected, then verify that an additional build type can be used with ros2 pkg create without changing those existing types.