Add a flag to write out models on solve

[odow]

As part of https://github.com/jump-dev/open-energy-modeling-benchmarks, I think it would be super useful to be able to write out all models that are solved to MPS files.

Then we don't have to instrument the various packages. We just set a flag in HiGHS.jl and collect them all.

cc @joaquimg @jd-lara

[odow]

The value should be a directory, and the filename should be the SHA of the file. So then we don't write out duplicates.

[odow]

Actually, I just abused type piracy:
https://github.com/jump-dev/open-energy-modeling-benchmarks/blob/1537ce1f20abe50bbba2dda7a465c8df53d0cf04/GenX/main.jl#L71-L75

I'll wait to see if we actually need this before implementing.

[odow]

I'm going to close this for now, but I might come back to it.