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.

modules/webhook/type.go

@@ -26,6 +26,7 @@ const (
 	HookEventPullRequestReviewRejected HookEventType = "pull_request_review_rejected"
 	HookEventPullRequestReviewComment  HookEventType = "pull_request_review_comment"
 	HookEventPullRequestSync           HookEventType = "pull_request_sync"
+	HookEventPullRequestReviewRequest  HookEventType = "pull_request_review_request"
 	HookEventWiki                      HookEventType = "wiki"
 	HookEventRepository                HookEventType = "repository"
 	HookEventRelease                   HookEventType = "release"
@@ -46,7 +47,7 @@ func (h HookEventType) Event() string {
 	case HookEventIssues, HookEventIssueAssign, HookEventIssueLabel, HookEventIssueMilestone:
 		return "issues"
 	case HookEventPullRequest, HookEventPullRequestAssign, HookEventPullRequestLabel, HookEventPullRequestMilestone,
-		HookEventPullRequestSync:
+		HookEventPullRequestSync, HookEventPullRequestReviewRequest:
 		return "pull_request"
 	case HookEventIssueComment, HookEventPullRequestComment:
 		return "issue_comment"