Add support for Highs (MI)LP solver

[sjunges]

The Highs MILP solver is freely available under an MIT license, hosted on github, and significantly seems to outperform GLPK, which is our current default. This makes it an interesting addition to our backends.

[sjunges]

I have also realised that GLPK has not seen updates in quite some time, which makes it even more interesting to switch/add a well-maintained LP solver.