Turning ROS2-For-Unity into a package

[RobertoRoos]

Relating to #86 . It doesn't seem nice ROS2ForUnity is locked into the Assets/ directory, when for all intends and purposes it is a package. So I'd suggest we make it installable as an actual package.

The most significant addition is just a package.json file.

[Deric-W]

Hi, I had a similar idea which I implemented in my fork but with some changes:

• since packages can contain dedicated examples I moved the current examples
• make Unity take care of packaging the metadata xml by putting it in a Resources directory
• make Unity take care initializing the PATH and checking the Ros Version using RuntimeInitializeOnLoadMethod

If you want to implement some of these changes too I can rebase them on the current upstream since currently they are in a private repository together with other changes.

[RobertoRoos]

Sounds great @Deric-W ! I would suggest to make your own PR and we can probably drop mine. I didn't make any changes on purpose besides just getting the packaging stuff to work.