Make unitree_joint_control_tools header-only.

[Masa0u0]

The following warning appeared when using the UnitreeJointController plugin:

SEVERE WARNING!!! A namespace collision has occurred with plugin factory for class unitree_legged_control::UnitreeJointController. New factory will OVERWRITE existing one. This situation occurs when libraries containing plugins are directly linked against an executable (the one running right now generating this message). Please separate plugins out into their own library or just don't link against the library and use either class_loader::ClassLoader/MultiLibraryClassLoader to open.

To address this, I made the unitree_legged_control_tool header-only and included only one source file in the plugin.