Add custom CI variable OPTIMIZATION_LEVEL

This allows for selection of an optimization level (-O0, -O1, ...) for building the BASOP code in the Gitlab UI when triggering a manual pipeline.

Why? We frequently see that MRs introduce regressions which are only detected later in the daily runs of the LTV regression checks. For critical changes, we run the LTV regression test manually for that branch, but it takes very long and regressions can also be introduced by changes that seem rather safe. Ideally, one would like to test LTV regressions for every merge. Using an optimized build for that would lower the hurdle of this more thorough testing. It might not be BE for all cases, but should be good enough for most cases and reveal most problems already (especially opposed to not running it at all). This MR adds the technical basis, one should agree on the procedure in next technical call.

About BE-ness with optimization: On one of our runners (with the conformance reference platform setup), stv testset is BE between -O0 and -O2. LTVs still to be checked.