Create a separate Steady Time datatype instead of having it embedded as an enumeration of time types

[tfoote]

Steady Time is a fundamentally different type than ROS Time and giving it it's own type will allow us to leverage compile time checking of the datatype instead of runtime. For steady time it can also contain logic to compare and verify the epoch ID for any other steady time to make sure you're comparing across the same epoch.

There's a related issue to create a message to represent it as well: ros2/rcl_interfaces#104

This has been discussed during development and was flagged during the API review for followup. ros2/common_interfaces#86