Improve webhook triggering/usage

On version 9.0.0 and earlier versions of Squash TM

General operation of webhooks on GitLab synchronizations:
when activated for a project, webhooks enable real-time updates of synchronized requirements whenever a modification occurs on an issue within the synchronization scope.

Behavior observed :
Webhooks are triggered between two synchronization cycles (during the configured sync time interval).

When a synchronization cycle is long (numerous synchronizations and/or large volumes of data to be updated) + the synchronization delay is short, the interest of webhooks is lost.

Food for thought :

webhooks for a given project are triggered once all the synchroinsations for that project have passed for a given cycle (and not wait until the end of the cycle for all sycnhronizations).
manage the squash.external.synchronization.delay property with a cron expression
watch out for the impact on performance of 'all the time' real-time updates
on the positive side, 'classic' update cycles might be less dense?

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