Make gitea webhooks openproject compatible (#28435)

This PR adds some fields to the gitea webhook payload that
[openproject](https://www.openproject.org/) expects to exists in order
to process the webhooks.
These fields do exists in Github's webhook payload so adding them makes
Gitea's native webhook more compatible towards Github's.

modules/structs/issue.go

@@ -30,6 +30,7 @@ type PullRequestMeta struct {
 	HasMerged        bool       `json:"merged"`
 	Merged           *time.Time `json:"merged_at"`
 	IsWorkInProgress bool       `json:"draft"`
+	HTMLURL          string     `json:"html_url"`
 }
 
 // RepositoryMeta basic repository information