Enhance the XML schema with metadata

[donmendelson]

An SBE message schema should have a standardized way to tell about the artifact--who issued it, when it was published, any copyright or license information, and so on. Fortunately, there are industry standard grammars that can be used for that purpose. The leading metadata candidates that have XML schemas are:

• Dublin Core Terms is standardized as ISO 15836. (FIX Orchestra XML schemas have a metadata section expressed as Dublin Core.)
• PROV is a W3C standard for provenance. It is more process oriented than Dublin Core.

For backward compatibility, the metadata section could be optional.

[donmendelson]

Proposing to use Dublin Core Terms in the same fashion as the Orchestra schema. It can be optional usage. If rules of engagement are expressed in Orchestra or other document type, metadata terms "references" or "relation" can be used to tie resources.