feat: Implemented a timer manager that can handle any timer type

[jmachowinski]

This is a draft commit as a basis for advancing the current timer manager implementation used by the events executor.

@alsora @fujitatomoya

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/the-ros-2-c-executors/38296/13

[alsora]

This implementation with a multimap rather than a priority queue is a lot simpler, but I would be curious to see the performance difference with respect to the current one.

IMO the current approach has benefits only when you have a lot of timers, but I doubt that executors will usually manage more than 10 timers each!