API: Add createdAt and updatedAt to api/users/lookup (#19496)

* API: Add `createdAt` and `updatedAt` to api/users/lookup

In the past, we have added both `updatedAt` (#19004) and `createdAt` (#19475) to /api/users/:id

Turns out, api/users/lookup uses the same DTO for both. This fixes the serialization of both `createdAt` and `updatedAt`for this endpoint.

Also, adds a test to ensure no further regressions.

* Updated API documentation

pkg/api/user.go

@@ -57,6 +57,8 @@ func GetUserByLoginOrEmail(c *m.ReqContext) Response {
 		Theme:          user.Theme,
 		IsGrafanaAdmin: user.IsAdmin,
 		OrgId:          user.OrgId,
+		UpdatedAt:      user.Updated,
+		CreatedAt:      user.Created,
 	}
 	return JSON(200, &result)
 }