Refactor JSON-LD context to separate repository

[theferrit32]

This will enable referring to the context via a remote URL in JSON-LD messages, instead of inlining it within each.

The compaction step in genegraph will need to fetch this file.

Should think about whether to put the context URL in an environment variable or hardcoding in source or one of the configuration resource files like kafka.edn. Different model->jsonld interceptor implementations might want different contexts depending on what the data is.