Standard conic form

The conic form we have is the geometric one, we should have a standard one too (i.e. variables in cones and equality constraints).

Start by reviewing the existing geometric conic form in MatrixOptInterface.jl and how the interface represents matrix-form problems. Determine the requirements for a standard form with variables in cones and equality constraints; the work is done when that form is represented and exposed consistently by the interface.

Written by the indexing model from the issue text.