mlos_bench: improve relative paths in configs

[bpkroth]

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It might be nice if the path resolution logic for a config allowed for paths relative to that config file (e.g. to reference scripts within that directory) as opposed to the full path within either the repo or the search path setup by the user config.

Forked off from #313

[bpkroth]

Added some initial work on this here: https://github.com/microsoft/MLOS/compare/main...bpkroth:local-path-resolution-improvements, but it will require some signature changes in many places, so will come back to it another time.