Implemented ROS actions

[RobertoRoos]

To fix #60

Currently not nearly done, but I figured it could be useful to already show the work.

[RobertoRoos]

Currently I got some extra Action related C# code to generate, but it looks like this new code is not actually compiled into the interface libraries. I think I'm missing some extra include somewhere.

@Deric-W maybe you know? How and where is the generated interface code used? Thanks!

EDIT: Found it by looking at #36 - I needed to list the files in the CMake.

[RobertoRoos]

I was trying to create a wrapper for the C-code action, with #36 as an example, but frankly I cannot reproduce this.

I might be able to create an action server instead using the existing implementations of services and messages in C#, like I'm doing here: https://github.com/RobotecAI/ros2cs/pull/61/files#diff-d8005b8b0c1727e0822794f32be7cacc0add2f27da8ab5958442a3008fd27ddfR58

@pijaro (or another maintainer), would you still be interested in merging something like this?