Event metadata format

Leverage map style metadata, used in cloud events that propagate as HTTP/message headers.

Having event metadata as JSON sounded like a good idea at a time, but it creates a lot of friction when things need to be retrieved or added, like tracing headers and other things. Header-style metadata is a well-known mechanism to avoid all the serialisation burden, and ESDB would benefit from adopting the same way to store and pass over metadata.