Add `GeometryId` to meshcat path API to `MeshcatVisualizer`

[nepfaff]

Is your feature request related to a problem? Please describe. This would be useful for changing the meshcat properties of a given Drake geometry. See this StackOverflow post for context.

Describe the solution you'd like MeshcatVisualizershould have an API (e.g. get_meshcat_path) that takes a GeometryID and returns the corresponding meshcat path name.

[nepfaff]

@SeanCurtis-TRI

[jwnimmer-tri]

Specifically, we the logic in MeshcatVisualizer<T>::SetObjects that converts a GeometryId and its FrameId into a path to be factored out and callable as a public function, something like std::string MeshcatVisualizer<T>::GetMeshcatPath(GeometryId) const;.