Allow metadata to be added to stored events

[tobyclemson]

It would be nice to have a separate record of arbitrary metadata against stored events, to capture data similarly to the payload but for non-domain specific data.

Examples of the kind of data that it would be good to capture are things like:

• tenancy information for multi-tenanted systems
• access control information in terms of event ownership
• causation relationships between events
• tracing and correlation identifiers to trace requests through an evented system
• tags or labels on events
• cryptographic hashes or signatures