Harmonise APIs for Entity\SmsMessageInterface and Message\SmsMessageInterface

Basically, this is saying that the Interfaces for Entity\SmsMessageInterface and Message\SmsMessageInterface should be the same as much as possible for ease of use. Only the internal implementation / storage should differ.

Two comments on SmsProviderInterface stand out.

  /**
   * Queue a standard SMS message for receiving.
   *
   * @todo Remove if standard message gets a direction property.
   *
   * @param \Drupal\sms\Message\SmsMessageInterface $sms_message
   *   A standard SMS message.
   */
  public function queueIn(SmsMessageInterface $sms_message);

  /**
   * Queue a standard SMS message for sending.
   *
   * @todo Remove if standard message gets a direction property.
   *
   * @param \Drupal\sms\Message\SmsMessageInterface $sms_message
   *   A standard SMS message.
   */
  public function queueOut(SmsMessageInterface $sms_message);

We should tidy up that @todo