Make loading of MonSvcInfoService optional and add support for schema files

Make the loading of the IS/OH publishing library optional in case the "monitoring" key is not available in the configuration dictionary. This is relevant for running in athenaHLT without OH/IS and ensures that hltinterface::IInfoRegister::instance() returns nullptr in that case.

Also add 'schema_files' options to the global configuration dictionary. This can be used to specify additional IS schema files to be given to the RDB server.

This has no impact for online running.

Relates to ATR-24502.

Also contains some unrelated/general python code cleanup.