func (i *IE) UserID() (*UserIDFields, error) {
	if i.Type != UserID {
		return nil, &InvalidTypeError{Type: i.Type}
	}
	s, err := ParseUserIDFields(i.Payload)
	if err != nil {
		return nil, err
	}
	return s, nil
}
UserID returns UserID in structured format if the type of IE matches.
package ie
import (
	"io"
	"github.com/wmnsk/go-pfcp/internal/utils"
)
func NewUserID(flags uint8, imsi, imei, msisdn, nai string) *IE {
	fields := NewUserIDFields(flags, imsi, imei, msisdn, nai)
	b, err := fields.Marshal()
	if err != nil {
		return nil
	}
	return New(UserID, b)
}