Vendored dependencies outside of the 3rdparty subfolder

[ericriff]

On #1012 I added options to opt out of all the vendored dependencies that live on the 3rdparty folder.

I just realized there are 4 more vendored dependencies, those live in include/behaviortree_cpp/contrib.

Is there a reason for them to be there instead on 3rdparty? Would you accept a PR where I move them to the 3rdparty folder and add options to opt out of them and use system/conan packages instead?

I'm willing to do the work, I just want to know if it is desired of it is the way it is for a reason.

Thanks.

[facontidavide]

So, those are more cmplicated.

Whilst the other dependencies are private ones, these, if I remember correctly are public.

This means that when dstributing the pre-compiled library (see ROS packages), the user will need them to compile their code.

I really don't want that

[ericriff]

I see. Closing this then :)