Examples with Solver Callbacks

Hi !

I am a big fan of PuLp and use it (almost) every day. I think it would be great if there were some examples showing how to use User Callbacks, such as Lazy Constraints or User Cuts.

This is done in the Jump package (Julia) for example, as shown in the docs [here](https://jump.dev/JuMP.jl/v0.12/callbacks.html).

I think this is possible, as per [this conversation](https://groups.google.com/g/pulp-or-discuss/c/gaDOaCdUSiQ/m/jjYPuqgbS0MJ) in pulp-or-discuss.

Start by reviewing the linked pulp-or-discuss conversation and the JuMP callback documentation to determine which solver callbacks PuLP supports. Done means adding clear examples for lazy constraints and user cuts, with any necessary usage limitations documented.

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