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/forms/repo_form.go

@@ -228,30 +228,31 @@ func (f *ProtectBranchForm) Validate(req *http.Request, errs binding.Errors) bin
 
 // WebhookForm form for changing web hook
 type WebhookForm struct {
-	Events               string
-	Create               bool
-	Delete               bool
-	Fork                 bool
-	Issues               bool
-	IssueAssign          bool
-	IssueLabel           bool
-	IssueMilestone       bool
-	IssueComment         bool
-	Release              bool
-	Push                 bool
-	PullRequest          bool
-	PullRequestAssign    bool
-	PullRequestLabel     bool
-	PullRequestMilestone bool
-	PullRequestComment   bool
-	PullRequestReview    bool
-	PullRequestSync      bool
-	Wiki                 bool
-	Repository           bool
-	Package              bool
-	Active               bool
-	BranchFilter         string `binding:"GlobPattern"`
-	AuthorizationHeader  string
+	Events                   string
+	Create                   bool
+	Delete                   bool
+	Fork                     bool
+	Issues                   bool
+	IssueAssign              bool
+	IssueLabel               bool
+	IssueMilestone           bool
+	IssueComment             bool
+	Release                  bool
+	Push                     bool
+	PullRequest              bool
+	PullRequestAssign        bool
+	PullRequestLabel         bool
+	PullRequestMilestone     bool
+	PullRequestComment       bool
+	PullRequestReview        bool
+	PullRequestSync          bool
+	PullRequestReviewRequest bool
+	Wiki                     bool
+	Repository               bool
+	Package                  bool
+	Active                   bool
+	BranchFilter             string `binding:"GlobPattern"`
+	AuthorizationHeader      string
 }
 
 // PushOnly if the hook will be triggered when push