Add handling of optimizer kwargs to mincost_flow

[mathieu17g]

In mincost_flow function, when trying to pass args to the optimizer via a closure, I hit a JuMP deprecation warning.
With the example from the documentation:

julia> flow = mincost_flow(g, demand, capacity, cost, () -> Clp.Optimizer(LogLevel=0))
┌ Warning: Passing optimizer attributes as keyword arguments to
│ Clp.Optimizer is deprecated. Use
│     MOI.set(model, MOI.RawParameter("key"), value)
│ or
│     JuMP.set_optimizer_attribute(model, "key", value)
│ instead.
└ @ Clp ~/.julia/packages/Clp/nnuI6/src/MOI_wrapper/MOI_wrapper.jl:42
6×6 SparseArrays.SparseMatrixCSC{Float64, Int64} with 6 stored entries:
  ⋅    ⋅    ⋅   1.0   ⋅    ⋅ 
  ⋅    ⋅   1.0   ⋅    ⋅    ⋅ 
  ⋅    ⋅    ⋅    ⋅    ⋅   1.0
  ⋅    ⋅    ⋅    ⋅    ⋅   1.0
 1.0  1.0   ⋅    ⋅    ⋅    ⋅ 
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅ 

Adding optimizer_kwargs... in the function arguments, and using JuMP.set_optimizer_attribute to pass them to the model, we avoid the deprecation.
I have not tested with any other optimizer than Clp