Update code generation to use ConstStringBase where appropriate

[ianbrault]

Work on nasa/fprime#4269 created a new immutable ConstStringBase string interface out of the original StringBase. This can be used in auto-generated FPP code, such as events, telemetry, and other miscellaneous port calls. It was decided that the autocoder update would require too much work to package with the initial effort so this issue is meant to track that.