Add API endpoint to get changed files of a PR (#21177)

This adds an api endpoint `/files` to PRs that allows to get a list of changed files.

built upon #18228, reviews there are included
closes https://github.com/go-gitea/gitea/issues/654

Co-authored-by: Anton Bracke <anton@ju60.de>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/swagger/repo.go

@@ -254,6 +254,28 @@ type swaggerCommitList struct {
 	Body []api.Commit `json:"body"`
 }
 
+// ChangedFileList
+// swagger:response ChangedFileList
+type swaggerChangedFileList struct {
+	// The current page
+	Page int `json:"X-Page"`
+
+	// Commits per page
+	PerPage int `json:"X-PerPage"`
+
+	// Total commit count
+	Total int `json:"X-Total"`
+
+	// Total number of pages
+	PageCount int `json:"X-PageCount"`
+
+	// True if there is another page
+	HasMore bool `json:"X-HasMore"`
+
+	// in: body
+	Body []api.ChangedFile `json:"body"`
+}
+
 // Note
 // swagger:response Note
 type swaggerNote struct {