Turning ROS2-For-Unity into a package

[Deric-W]

Hi, this pull request has the same goal as #94 but with the following extra features:

• ros2-for-unity.xml is now being packaged by Unity
  • postinstall now only handles versioned shared libraries
  • the metadata file is moved into a resource folder, which seems to be discouraged but since it is only a small xml the addressables would probably be overkill
• the path and integrity checks are now done by Unity (implies some simple changes when running in edit mode)
  • some methods of ROS2ForUnity where moved to a separate Setup class and marked private or internal where possible
  • since Unity does not call RuntimeInitializeOnLoadMethod marked methods in edit mode the user has to call the method to setup the path manually in this case (for example using OneTimeSetUp in NUnit tests)
• examples where moved to dedicated package examples
  • they have dedicated scenes which meant the example image could be removed
• moved Scripts to Runtime directory and add Assembly definition
  • as a side effect internal methods and attributes are now hidden from users
  • some missing .meta files where added but the generated binaries still need to be imported from disk at least once before uploading the package to a registry to generate .meta files for them

If these features are wanted then this PR could replace #94.

[RobertoRoos]

Looks neat @Deric-W ! Until this gets merged I'll be basing my own forked works on this.

EDIT: Maybe, as the diff is pretty huge, you could complete the list in your first message with all the changes made? I imagine bullets like 'moved Scripts to Runtime' and 'replaced path variables by Resources helper', etc.

[RobertoRoos]

@Deric-W I've started using your branch in my own project ~~and it looks like I'm having issues with custom messages. I can compile fine, custom messages are recognized properly. But when running I get UnsatisfiedLinkErrors for DLLs like [message]__rosidl_typesupport_c_native.dll. Do you think you could give it a try too?~~

EDIT: Oops, fault was mine. Can confirm custom message still work fine as a package!