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.

routers/web/repo/issue.go

@@ -576,7 +576,7 @@ func RetrieveRepoReviewers(ctx *context.Context, repo *repo_model.Repository, is
 	}
 	ctx.Data["OriginalReviews"] = originalAuthorReviews
 
-	reviews, err := issues_model.GetReviewersByIssueID(issue.ID)
+	reviews, err := issues_model.GetReviewsByIssueID(issue.ID)
 	if err != nil {
 		ctx.ServerError("GetReviewersByIssueID", err)
 		return