Add support for quadratic objective functions

I am trying to use Convex.jl to interface with the OSQP solver. However, for some reason I am unable to use quadform in the objective and still have it link to the OSQP optimizer.
```julia
using Convex
using LinearAlgebra
using OSQP

x = Variable(5)
P = Diagonal(ones(5))

problem = minimize(quadform(x, P))

solve!(problem, OSQP.Optimizer)
```
When this code runs I get the following error:
```
LoadError: MathOptInterface.UnsupportedConstraint{MathOptInterface.VectorAffineFunction{Float64},MathOptInterface.SecondOrderCone}("")
```

I don't understand why it throws an unsupported constraint error as there are no constraints.