Enh: Supporting a custom mapping in setup

Hi,

Currently, `tools/setup.py` has a hard-coded mapping in code. In my use-case, I need some extra fields per message, so I think it'll be useful to allow the user provide a command-line option, which will read an external JSON file with the mapping. We can have a default mapping, same as the current one. Would you consider accepting such a PR?

Start in tools/setup.py and inspect the existing hard-coded mapping and command-line handling. Define how an external JSON mapping is selected, how the current mapping remains the default, and how extra per-message fields are preserved. Done means setup accepts the new option while continuing to work unchanged without it.

Written by the indexing model from the issue text.