potobuilder: add Clone method to all builders

[Smithx10]

I Am trying to set the same field for a req and response but only 1 gets set. I have to do the following for it to work.
Doesn't Work:

			case "Create":
				// Res
				if err := req.TryAddField(tf); err != nil {
					return err
				}
				// Res
				if err := res.TryAddField(tf); err != nil {
					return err
				}

Always have to create a New Field:

			case "Create":
				// Res
				if err := req.TryAddField(protobuilder.NewField(tf.Name(), tf.Type())); err != nil {
					return err
				}
				// Res
				if err := res.TryAddField(protobuilder.NewField(tf.Name(), tf.Type())); err != nil {
					return err
				}

[jhump]

@Smithx10, I see the documentation isn't particularly clear on this. But this is, in fact, expected. Each builder also includes a link up to its parent. If you add a field to another message, it changes its parent.

I think a proper solution to the issue you face will be twofold:

1. Updates docs to make it more clear that adding an element to a new parent moves the element; it doesn't copy it nor can multiple builders share references to the same child element.
2. Provide a Clone() method on each builder type, to make it trivial to copy them, for cases like this.