add admin API email endpoints (#22792)

add email endpoint to admin API to ensure API parity with admin
dashboard.

modules/structs/user_email.go

@@ -1,4 +1,5 @@
 // Copyright 2015 The Gogs Authors. All rights reserved.
+// Copyright 2023 The Gitea Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
 package structs
@@ -9,6 +10,8 @@ type Email struct {
 	Email    string `json:"email"`
 	Verified bool   `json:"verified"`
 	Primary  bool   `json:"primary"`
+	UserID   int64  `json:"user_id"`
+	UserName string `json:"username"`
 }
 
 // CreateEmailOption options when creating email addresses