Add option to make `MOIU.@model` and AbstractOptimizer

[odow]

Currently, @model makes an AbstractModel subtype
https://github.com/JuliaOpt/MathOptInterface.jl/blob/fb303dceaa85dfff8f97e7fe13542c70fdef7754/src/Utilities/model.jl#L927
where AbstractModel is
https://github.com/JuliaOpt/MathOptInterface.jl/blob/fb303dceaa85dfff8f97e7fe13542c70fdef7754/src/Utilities/model.jl#L85

However, solvers like PATH.jl and AmplNLWriter use @model as an AbstractOptimizer. We should add a flag to let the super type be chosen by the constructor.

Alternatively, is there any reason not to make AbstractMode{T} <: MOI.AbstractOptimizer?

Re-reading, #203, it's not obvious why we have the ModelLike/AbstractOptimizer distinction.

This feels like it's a legacy of the MPB solver/instance distinction.

cc @chkwon

[blegat]

I have no objection for adding an option @model, it can even be part of a non-breaking release if we add a default value.
Removing ModelLike would be a separate discussion since it is a breaking change. Not sure what to think about this one.

[odow]

Closed by #1031