[wpiutil] Struct: Change from GetTypeString() to GetTypeName() (#6872)

This makes it easier to define schemas when the type name is non-trivial (e.g., templated structs).

This is breaking for a) custom struct implementations and b) anything calling `wpi::Struct<T>::GetTypeString(info...)` in C++ directly. In both cases, it's a simple translation: For A, rename `GetTypeString()` to `GetTypeName()` and remove the struct: at the beginning, and for B, use `wpi::GetStructTypeString<T>(info...)` instead.

wpimath/src/main/native/include/frc/geometry/struct/Rotation3dStruct.h

@@ -11,9 +11,7 @@
 
 template <>
 struct WPILIB_DLLEXPORT wpi::Struct<frc::Rotation3d> {
-  static constexpr std::string_view GetTypeString() {
-    return "struct:Rotation3d";
-  }
+  static constexpr std::string_view GetTypeName() { return "Rotation3d"; }
   static constexpr size_t GetSize() {
     return wpi::GetStructSize<frc::Quaternion>();
   }