Add Batched{S} set

[odow]

@amontoison wants this. There are also some folks in the GPU/cvxpy domain sniffing around this (cvxpy/cvxpy#2485).

We have almost everything we need. There's just a choice between two approaches.

First, we could set the value of Parameter to a vector of sets:

MOI.set(
    ::Optimizer,
    ::MOI.ConstraintSet,
    ::MOI.ConstraintIndex{MOI.VariableIndex,MOI.Parameter{T}},
    ::Vector{MOI.Parameter{T},
)

But this might be hard to get through the various MOI layers.

The easier alternative is to add a new Batched{S<:MOI.AbstractSet} set.

If there are multiple batched sets, they all must have the same length. Then the results are returned via result_count and we don't support returning multiple solutions within a batch.

Then a solver could choose to natively support batched parameters. And we could have a fallback optimizer like:

using JuMP
import Ipopt
import MathOptBatchOptimizer as MOBO
model = Model(() -> MOBO.Optimizer(Ipopt.Optimizer))
@variable(model, x)
@variable(model, p in Parameter(1))
set_parameter_value(p, 1:100) # <-- set as a vector, not a scalar
@objective(model, Min, x)
@constraint(model, x >= p)
optimize!(model)
@assert result_count(model) == 100