Highs mods

[jajhall]

Here are almost all the modifications that I've made so that cuPDLP-C can be run from HiGHS. The issues that I've had are set out in README.md. The modifications have been written so that the original cuPDLP-C behaviour is retained when these files are used in https://github.com/COPT-Public/cuPDLP-C

The changes that I've not included are those relating to cmake, since I can't code them so that the original cuPDLP-C behaviour is retained. I'll simply have to re-do them as/when I pull and changes that are made to https://github.com/COPT-Public/cuPDLP-C

[jajhall]

When adding two more settings to INT_USER_PARAM, I got CI failures until I spotted that N_INT_USER_PARAM and N_FLOAT_USER_PARAM are set explicitly rather than implicitly as the last entry in the enum. Doing the latter seems to be much better practice

[bzhangcw]

Seem to be problematic with the GPU version. Maybe stage it as a new branch and merge it later? @SkyLiu0