[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/java/edu/wpi/first/math/system/plant/struct/DCMotorStruct.java

@@ -15,8 +15,8 @@ public class DCMotorStruct implements Struct<DCMotor> {
   }
 
   @Override
-  public String getTypeString() {
-    return "struct:DCMotor";
+  public String getTypeName() {
+    return "DCMotor";
   }
 
   @Override