Add example for external optimizer/benchmark

[benjamc]

• Add hydra search path to command: python -m carps.run 'hydra.searchpath=[pkg://yourpackagename/maybesomesubfolder/configs,pkg://someotherpackage/configs]' +experiment=base +optim=heboextended +problem/OptBench=synth_Ackley_5 'seed=644'
• Package needs to be installed via pip such that hydra can find it
• Package contains configs folder and subfolder, just like carps. Beware (not sure about): do not name the subfolders in the configs the same way as in carps, maybe the configs cannot be found then