Unwrap for ValidationError

[kdeberk]

This PR adds a single Unwrap function to the ValidationError.

With this function, it is possible to call errors.Is(err, target) or errors.As(err, &target) with the error returned by e.g. ParseWithClaims.

More specifically, it allows one to replace

       _, err := jwt.ParseWithClaims(...)

	var valErr *jwt.ValidationError
	if errors.As(err, &valErr) {
		if errors.Is(valErr, myErrorType) {
			// do X
		} else {
                        // do Y
                }
	}

with

       _, err := jwt.ParseWithClaims(...)
       if errors.Is(err, myErrorType) {
           // do X
       }  else {
           // do Y
       }