Add MQTT message expiry interval

[dimabarbul]

Resolves #1729

Please, be patient. As I'm C# developer, I assume, there are moments done in non-Java way. If you find some names or language constructs that feel strange or non-Java-like, I'd be grateful if you point them out to me.

My concerns are:

1. In class ExternalMessageToMqttPublishTransformerTest constant RETAIN_FALL_BACK was not used. It might have been used in method:

   @Test
   public void transformExternalMessageWithoutRetainValueInHeadersFallsBackToFalse() {
       Mockito.when(externalMessage.getHeaders()).thenReturn(DittoHeaders.empty());
   
       assertThat(ExternalMessageToMqttPublishTransformer.transform(externalMessage, mqttPublishTarget))
               .isEqualTo(TransformationSuccess.of(externalMessage,
                       GenericMqttPublish.builder(MQTT_TOPIC_FALL_BACK, MQTT_QOS_FALL_BACK)
                               .retain(false)
                               .build()));
   }

   I believe, as long as the method is named as "*FallsBackToFalse" it's ok to use false instead of class constant.

2. I'm not sure about implementation of class MessageExpiryInterval, especially about method getAsOptionalLong. I was choosing between Optional<Long> and OptionalLong. I chose OptionalLong because it doesn't do boxing and it has only 2 states - does or doesn't have value, whereas Optional<Long> allows 3 states - doesn't have value, has null value and has not-null value.

3. Header name mqtt.message-expiry-interval seems a bit long. I thought about mqtt.expiry but decided to stick with MQTT 5 name.

4. In class GenericMqttPublish I didn't know what to do when we got message from broker with invalid message expiry interval. I decided to not throw any exception and just fall back to no message expiry interval. Probably, need to log something here. Throwing exception doesn't seem to be an option here, cause it'll be needed to be added to signatures of whole call tree of this method.

5. I added MQTT version to enum MqttHeader. Not sure if adding this kind of info to enum is OK in general.

6. I consider adding message expiry interval to last will message as out-of-scope.

[thjaeckle]

Thanks a lot for the contribution @dimabarbul
I will try to look at it soon 👍

[thjaeckle]

Hey @dimabarbul
Looks really good, thanks a lot. 👍
I only added a few comments, mainly about missing javadoc on public methods.

[thjaeckle]

LGTM, thanks for the contribution @dimabarbul 👍