Test usability of HiGHS solver through `GAMSModel`

[khaeru]

This PR tests that the free and open source HiGHS solver can be used through ixmp.

The GAMSModel class is enhanced to allow passing an LP=… keyword argument (thus also Scenario.solve(…, LP=…)) to enable selection of this, or other, solvers.

It appears to be not currently possible to test this on GitHub Actions:

• The HiGHS solver is only packaged with GAMS 40 and later versions.
• More recent versions of GAMS do not allow any use without a license.
  • A "free community license" is available by e-mail request; we could possibly request one of these and use it for ixmp CI.
  • Or, we could instead add support for HiGHS via another AML like Pyomo (#420) or Linopy.

How to review

• Read the diff and release notes.
• Note the CI checks all pass.

PR checklist

• [ ] Continuous integration checks all ✅

• [ ] Add or expand tests; coverage checks both ✅
• [ ] Add, expand, or update documentation.

• [ ] Update release notes.