Update CQM serialization to allow variable data to be encoded outside of the objective

[arcondello]

Right now, the CQM serialization tracks the variable types and bounds in the objective. This is a legacy from dimod<0.12.0 where the CQM itself tracked them that way. These days, the objective can also be sparse, but we've kept the serialization.

We should create a new CQM serialization version 1.4 that stores the variable info globally so that the objective can be sparse or even not present.