refactor to support msg inside msg, constant, static array initialize

[yuokamoto]

• use msg inside msg instead of expanding all msgs
• Add namespace as prefix for msg name. Prefix no need to be same as original namespace and defined in yaml file.
• Add word conversion. Support conveting word such as Covariance -> Cov. This is used to avoid too long class/struct name to avoid breaking UE maximum class name. https://forums.unrealengine.com/t/qeustion-about-class-name-length-limitation-when-adding-code-through-the-editor/49552
• Support initializing fix sized array
• Define constant as constexpr and CONST_ function to used in BP.
• use functions in rclUtilities in rclUE

related pr: https://github.com/rapyuta-robotics/rclUE/pull/59