Build deploy hook payload in-request

What does this MR do and why?

This change builds the deploy webhook payload in the request, rather than delayed through Deployments::HooksWorker.

Once the payload is built, it is sent to the queue via WebHookWorker and the webhook executes on the queue, as normal.

This change is behind a feature flag to allow us to test whether the change will fix a problem for a particular customer who has some webhook payloads containing a "status" of "running" when the deployment has finished, when the status should be "succeeded".

• Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/355903

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #355903