Centralize parsing of JSON files to a single struct

The codebase contains several hard-coded behaviour regarding the format of the JSON files. Examples include:

• https://gitlab.cern.ch/lhcb/Allen/-/blob/master/main/src/Provider.cpp#L48
• https://gitlab.cern.ch/lhcb/Allen/-/blob/master/mdf/test/test_mep_banks.cpp#L89

These examples are hard to find and debug, and therefore any change to the JSON format is very painful at the moment. We should have a single entrypoint where all required functionality is defined, and always use that entrypoint.