Add a Dragway Factory To Simplify the Public API and Hide Details

[liangfok]

Quotes from #5038:

From @maddog-tri:

However, something that would be nice from the user's perspective is to provide a static constructor function (e.g., MakeDragway(...)) in its own header file which returns an api::RoadGeometry --- that way none of the other dragway header files even need to be installed anywhere. (Heck, it could all get buried into a single .cc file at that point, not that that necessarily improves maintainability.)

From @jwnimmer-tri:

Note that a good way to compact it (if you choose to do so) is to have only one public master outer class (possibly inheriting nothing, or possibly RoadGeometry) and then have an private Impl class within in implement all of the interfaces (rather than directly expose them to the public). That saves all of the syntax of separate declaration-and-definition because all of the signatures go directly into the cc file, with bodies.

[liangfok]

After integrating dragway with AutomotiveSimulator, I don't think the current API is cumbersome enough to require factory support. In addition, the exposure of dragway-specific Junction, Segment, and Lane APIs has not been problematic. I shall thus close this issue.