New webhook trigger for receiving Pull Request review requests (#24481)

close https://github.com/go-gitea/gitea/issues/16321

Provided a webhook trigger for requesting someone to review the Pull
Request.

Some modifications have been made to the returned `PullRequestPayload`
based on the GitHub webhook settings, including:
- add a description of the current reviewer object as
`RequestedReviewer` .
- setting the action to either **review_requested** or
**review_request_removed** based on the operation.
- adding the `RequestedReviewers` field to the issues_model.PullRequest.
This field will be loaded into the PullRequest through
`LoadRequestedReviewers()` when `ToAPIPullRequest` is called.

After the Pull Request is merged, I will supplement the relevant
documentation.

services/convert/pull.go

@@ -88,6 +88,14 @@ func ToAPIPullRequest(ctx context.Context, pr *issues_model.PullRequest, doer *u
 		},
 	}
 
+	if err = pr.LoadRequestedReviewers(ctx); err != nil {
+		log.Error("LoadRequestedReviewers[%d]: %v", pr.ID, err)
+		return nil
+	}
+	for _, reviewer := range pr.RequestedReviewers {
+		apiPullRequest.RequestedReviewers = append(apiPullRequest.RequestedReviewers, ToUser(ctx, reviewer, nil))
+	}
+
 	if pr.Issue.ClosedUnix != 0 {
 		apiPullRequest.Closed = pr.Issue.ClosedUnix.AsTimePtr()
 	}