Streamline creation of subscriptions in callback groups

Feature request

Feature description

The motivation behind this is that I would like a convenient way to create a subscription associated with a callback group within a node's initializer list. It is straightforward to do this for actions and services, since rclcpp::create_service, rclcpp_action::create_server, etc., allow me to pass a callback group as an argument. However, the rclcpp::create_subscription function takes a SubscriptionOptions struct as an argument, and the SubscriptionOptions struct does not provide a constructor that takes a rclcpp::CallbackGroup::SharedPtr. This means that I have to pass an instance of a CallbackGroup to my SubscriptionOptions in the body of the node's constructor and then create the subscription using those SubscriptionOptions.

Implementation considerations

A simple approach would be to add a constructor overload to SubscriptionOptions that just takes rclcpp::CallbackGroup::SharedPtr as an argument. As an extension, rclcpp::create_subscription could be overloaded to take a callback group as an argument, similar to the equivalent functions that create clients and servers.

Start with the SubscriptionOptions type and the rclcpp::create_subscription entry point described in the issue, then compare them with the callback-group arguments accepted by the client and server creation helpers. Determine whether initializer-list construction can support a callback group through SubscriptionOptions or an overload. Done means subscriptions can be associated with a callback group during node initialization, with the chosen API behavior covered by the relevant project tests.