This avoids needing to jump through type paremeter hoops to create a
sub-classable Cue.Builder.
Cue should have a standard set of fields, because it will be
consumed by renderers that don't know what type it is. The
existing subclass fields are only used inside their respective
packages, so can be part of a wrapper object instead.
This lays the groundwork for converting Cue's multiple constructors
into a Builder pattern.
PiperOrigin-RevId: 284992135
e8fa9a55a5