Add iteration_id and iteration_name to Webhooks Issue event

Release notes

Problem to solve

When receiving a Webhooks Issue event, the iteration_id and iteration_name are not part of the transmitted data (but milestone_id is). It is also impossible to retrieve the iteration_id through the Issues API, see #326747 (closed) .

This information would be useful for creating Webhooks dependent on the iteration an issue is assigned to or not (e.g. labeling, adding to a certain milestone, forcing a non-empty weight, etc.).

Proposal

Add the field to the WebHooks datastructure.