Rename *Base traits to *Waitable

[nnmm]

This is better in my opinion for a few reasons:

• "Base" suggests inheritance, but unlike in rclcpp, there is no inheritance in rclrs
• The functionality provided by e.g. SubscriptionBase in rclcpp and rclrs is very different, so using the same name for both suggests an equivalence that isn't there
• *Waitable is more descriptive, since the purpose of these traits is to interface subscriptions, services, and clients with wait sets