Consider a 'datasource' augmentation for pertinent schemas

[Athanasius]

Commodites data can come from either CAPI or Journal.

It seems that CAPI /market does not contain any rare commodities, but if a user causes Journal Market.json to be utilised at the same station, and it has a rare commodity for sale, the resulting message will include it. Naively a consumer might think the CAPI data means the rare is no longer available there.

For this, and other reasons, it would be useful for data consumers to know what data source was used to produce a message.

Propose values would be:

1. journal-pc - From a journal file written directly on a PC running the game client.
2. journal-capi - From a journal sourced from the CAPI /journal endpoint, which is known to sometimes be problematic.
3. capi - Data from other CAPI endpoints.

We should perhaps leave things open to list multiple values, but that will complicate things. It would be better to send multiple messages if using data from multiple sources.