geometry: Improve data representation of convex meshes in ProximityEngine

[DamrongGuoy]

We would like to improve data representation of convex meshes in ProximityEngine. The data is needed by fcl::Convex. The improvement includes

1. Change ConvexData from a struct to a shared_ptr of struct.
2. Change convex_objects_ from a vector of ConvexData to a map from filename to ConvexData.
3. Copy convex_objects_ in the copy constructor of ProximityEngine::Impl.

Improvement 1 is in anticipation of the future fcl::Convex constructor that will take shared_ptr.
Improvement 2 will optimize instantiation of multiple objects of the same shape from the same file.
Improvement 3 is to prevent a potential problem introduced by #9471.

[jwnimmer-tri]

FYI @SeanCurtis-TRI per your recent work on convex & fcl.

[jwnimmer-tri]

Actually, if we haven't cared about this in the past 6 years, I'll venture a guess that it's not important anymore, so closing as "not planned".

[SeanCurtis-TRI]

Ironically, it's in the midst of being kind of addressed with #21147.

Mesh and Convex now have a unique representation of their convex hull which has a direct mapping to fcl::Convexd. The only thing we're really missing is a geometry cache so we don't have to parse the same file over and over. But that's a known, long-lasting thing that doesn't need an issue.