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.

models/issues/pull.go

@@ -175,9 +175,10 @@ type PullRequest struct {
 
 	ChangedProtectedFiles []string `xorm:"TEXT JSON"`
 
-	IssueID int64  `xorm:"INDEX"`
-	Issue   *Issue `xorm:"-"`
-	Index   int64
+	IssueID            int64  `xorm:"INDEX"`
+	Issue              *Issue `xorm:"-"`
+	Index              int64
+	RequestedReviewers []*user_model.User `xorm:"-"`
 
 	HeadRepoID          int64                  `xorm:"INDEX"`
 	HeadRepo            *repo_model.Repository `xorm:"-"`
@@ -302,6 +303,29 @@ func (pr *PullRequest) LoadHeadRepo(ctx context.Context) (err error) {
 	return nil
 }
 
+// LoadRequestedReviewers loads the requested reviewers.
+func (pr *PullRequest) LoadRequestedReviewers(ctx context.Context) error {
+	if len(pr.RequestedReviewers) > 0 {
+		return nil
+	}
+
+	reviews, err := GetReviewsByIssueID(pr.Issue.ID)
+	if err != nil {
+		return err
+	}
+
+	if len(reviews) > 0 {
+		err = LoadReviewers(ctx, reviews)
+		if err != nil {
+			return err
+		}
+		for _, review := range reviews {
+			pr.RequestedReviewers = append(pr.RequestedReviewers, review.Reviewer)
+		}
+	}
+	return nil
+}
+
 // LoadBaseRepo loads the target repository. ErrRepoNotExist may be returned.
 func (pr *PullRequest) LoadBaseRepo(ctx context.Context) (err error) {
 	if pr.BaseRepo != nil {