[launch] refactor introspection system

Some effort was already invested to express intent without side-effects in the new launch api, and each entity in a launch description can express itself and related, or "sub" entities. However, the introspection interfaces of various entities (Actions, Event Handlers, Events, Subsitutions, etc...) need to be refined to avoid impassible and opaque boundaries in the description when introspecting.

I have some ideas of what needs to change already, but ran out of time before the bouncy release. The launch.LaunchDescriptionEntity interfaces for this are the most "mature" and reflect somewhat closely what I'd like to update the other entities (like EventHandler and Substitution) to look like in the future:

https://github.com/ros2/launch/blob/b78d87f31bb995c7226df66826e1a57079ae3a3f/launch/launch/launch_description_entity.py#L57-L101

The above interfaces may also need some work. Additionally, I think that right now the mechanism to "render" an entity during introspection (render to text for printing on the console in the case of launch.LaunchIntrospector) needs to be placed with or along side the entities themselves. Currently this logic is in the LaunchIntrospector, which was only done to get something working rapidly:

https://github.com/ros2/launch/blob/b78d87f31bb995c7226df66826e1a57079ae3a3f/launch/launch/launch_introspector.py#L72-L140

Lastly, specific entities need to implement these interfaces and handle any special logic to make them useful during introspection. Specifically I noticed:

• launch.EventHandlers could report what is yielded when an event is handled
• Update introspection support for some launch.Actions (e.g. the ExecuteProcess action could report more of what it might do in certain situations)

A stretch goal of this refactoring would be to add some notion of "details" when introspecting, e.g. you might just want to see that a process is getting run and not all the various and tedious things it might do in every situation, unless you asked for those details, in which case it would show them. Basically some way for an entity to express that some sub entities and parts of its own description are implementation details and not usually important to express.

Start by reading launch/launch/launch_description_entity.py, especially the referenced interfaces, and launch/launch/launch_introspector.py where rendering currently lives. Compare the introspection needs of EventHandlers, Substitutions, and Actions such as ExecuteProcess. Done means the interfaces and entity-specific introspection support are coherently refined, with rendering associated with the entities and useful output for the noted cases.