add admin API email endpoints (#22792)

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

routers/api/v1/api.go

@@ -1260,6 +1260,10 @@ func Routes(ctx gocontext.Context) *web.Route {
 					m.Post("/rename", bind(api.RenameUserOption{}), admin.RenameUser)
 				}, context_service.UserAssignmentAPI())
 			})
+			m.Group("/emails", func() {
+				m.Get("", admin.GetAllEmails)
+				m.Get("/search", admin.SearchEmail)
+			})
 			m.Group("/unadopted", func() {
 				m.Get("", admin.ListUnadoptedRepositories)
 				m.Post("/{username}/{reponame}", admin.AdoptRepository)