Clarify JSON Schema Version

[SteadBytes]

The SCHEMA message specification states that each message should be a valid JSON Schema. However, as far as I can tell, it is not documented which version of the JSON Schema specification Singer tools should support (drafts v4, v6, v7 e.t.c.).

If this is already decided on, is it worth adding to the SCHEMA message specification? Otherwise, should it be decided on? I'm having difficulty understanding the exact format of the various message types due to differing version constraints between different taps/targets (catalog vs properties, schema formats e.t.c.). Having the JSON Schema version specified would be a helpful start to knowing how a tap/target should be developed.

Thank you!