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/structs/pull.go

@@ -9,19 +9,20 @@ import (
 
 // PullRequest represents a pull request
 type PullRequest struct {
-	ID        int64      `json:"id"`
-	URL       string     `json:"url"`
-	Index     int64      `json:"number"`
-	Poster    *User      `json:"user"`
-	Title     string     `json:"title"`
-	Body      string     `json:"body"`
-	Labels    []*Label   `json:"labels"`
-	Milestone *Milestone `json:"milestone"`
-	Assignee  *User      `json:"assignee"`
-	Assignees []*User    `json:"assignees"`
-	State     StateType  `json:"state"`
-	IsLocked  bool       `json:"is_locked"`
-	Comments  int        `json:"comments"`
+	ID                 int64      `json:"id"`
+	URL                string     `json:"url"`
+	Index              int64      `json:"number"`
+	Poster             *User      `json:"user"`
+	Title              string     `json:"title"`
+	Body               string     `json:"body"`
+	Labels             []*Label   `json:"labels"`
+	Milestone          *Milestone `json:"milestone"`
+	Assignee           *User      `json:"assignee"`
+	Assignees          []*User    `json:"assignees"`
+	RequestedReviewers []*User    `json:"requested_reviewers"`
+	State              StateType  `json:"state"`
+	IsLocked           bool       `json:"is_locked"`
+	Comments           int        `json:"comments"`
 
 	HTMLURL  string `json:"html_url"`
 	DiffURL  string `json:"diff_url"`