Add iteration data to webhook payload

Problem To Solve

However, the most important issue that we are running into is that this information is missing from the webhook payload - specifically, the very webhook payload that is generated from a change to the iteration field itself...In other words, the webhook tells us that the iteration was changed, but does not tell us what it was changed to.

Ref: #327549 (comment 584753697)

Proposal

• Issue event webhook payload includes data for both the iteration data before the event and the event itself (the new iteration value).
• Add Iteration Events to group/project webhooks that fire when an iteration is created/mutated.

Changes need to happen around this file https://gitlab.com/gitlab-org/gitlab/-/blob/13b7598ac95fa214235a8c20339b8c59308693df/ee/lib/ee/gitlab/hook_data/issue_builder.rb. Iteration data should be added there.

Availability and Testing

Manual job package-and-qa will need to be run in the MR(s) that introduces the change.

Ensure test coverage in unit/integration tests.