SLING-9389: Distribution Event Packages should contain queue item creation time

[harshchiki]

• Improvement aims at adding the queue item creation time, essentially when the the item was creation for the first time, and enqueue into the queue.
• The purpose to get this detail is to be able to capture metrics at the consumer level. The consumers could have an event handler, which can capture the duration which turns out to be (NOW MINUS queue item creation time thrown in the distribution event package); NOW being the current time in the event handler (consumer).

[harshchiki]

SCD API PR - apache/sling-org-apache-sling-distribution-api#2

[harshchiki]

This PR is failing on the missing DistributionEventProperties.DISTRIBUTION_ENQUEUE_TIMESTAMP(compilation error), which is proposed in apache/sling-org-apache-sling-distribution-api#7 - IIUC, once that is merged, this should pass fine.

Request the owners of the repo to kindly review.

cc: @tmaret @cschneider