Support quadratic objective in conic form

[blegat]

See oxfordcontrol/COSMO.jl#126
This might be breaking as it changes the type

[matbesancon]

So we would represent the quadratic part with an abstract matrix right?

[blegat]

The objective would be an MOI.AbstractScalarFunction. The plan is to have matrix forms be parametrization of MOIU.GenericModel (see jump-dev/MathOptInterface.jl#1261).
It would be parametrized by

1. the objective type
2. the container for variable constraints
3. the container for other constraints
   Currently, in MOIU.GenericModel, the objective type is hardcoded as Union{MOI.SingleVariable, MOI.ScalarAffineFunction{T}, MOI.ScalarQuadraticFunction{T} however, this type should be a parameter of MOIU.GenericModel instead.
   Then, similarly to how https://github.com/jump-dev/MathOptInterface.jl/pull/1238 got ScalarAffineFunction generalized with the AbstactVector of terms, we could generalize ScalarQuadraticFunction with an AbstractMatrix of terms. One option could be to store the coefficients in a sparse matrix.

[matbesancon]

Given that in MatOI we take and store the elements of the optimization problem directly as matrices and vectors, I would be for the second option of having an abstract matrix