[API] generalize list header (#16551)

* Add info about list endpoints to CONTRIBUTING.md

* Let all list endpoints return X-Total-Count header 

* Add TODOs for GetCombinedCommitStatusByRef

* Fix models/issue_stopwatch.go

* Rrefactor models.ListDeployKeys

* Introduce helper func and use them for SetLinkHeader related func

routers/api/v1/user/gpg_key.go

@@ -28,6 +28,13 @@ func listGPGKeys(ctx *context.APIContext, uid int64, listOptions models.ListOpti
 		apiKeys[i] = convert.ToGPGKey(keys[i])
 	}
 
+	total, err := models.CountUserGPGKeys(uid)
+	if err != nil {
+		ctx.InternalServerError(err)
+		return
+	}
+
+	ctx.SetTotalCountHeader(total)
 	ctx.JSON(http.StatusOK, &apiKeys)
 }