[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.

wpiutil/src/main/java/edu/wpi/first/util/struct/Struct.java

@@ -47,13 +47,23 @@ public interface Struct<T> {
    */
   Class<T> getTypeClass();
 
+  /**
+   * Gets the type name (e.g. for schemas of other structs). This should be globally unique among
+   * structs.
+   *
+   * @return type name
+   */
+  String getTypeName();
+
   /**
    * Gets the type string (e.g. for NetworkTables). This should be globally unique and start with
    * "struct:".
    *
    * @return type string
    */
-  String getTypeString();
+  default String getTypeString() {
+    return "struct:" + getTypeName();
+  }
 
   /**
    * Gets the serialized size (in bytes). This should always be a constant.