Define Core term for service/schema/model version

Add a Core term for a service/schema/model version. <Term Name="SchemaVersion" Type="String" AppliesTo="Schema"> <Annotation Term="Core.Description" String="Version of the schema (not to be confused with the protocol version)" /> </Term> Clarify relationship between version and metadataEtag: first is semantic, second could be technically derived (timestamp of development artifacts), so the latter may change without the former changing, but if the former changes, the latter SHOULD/MUST change.

https://www.oasis-open.org/apps/org/workgroup/odata/download.php/59028/odata-v4.01-wd01-part1-protocol.docx https://www.oasis-open.org/apps/org/workgroup/odata/download.php/59031/odata-csdl-xml-v4.01-wd01.docx https://www.oasis-open.org/apps/org/workgroup/odata/download.php/59030/odata-json-format-v4.01-wd01.docx https://tools.oasis-open.org/version-control/browse/wsvn/odata/trunk/4.01%20spec/ABNF/odata-abnf-construction-rules.txt https://tools.oasis-open.org/version-control/browse/wsvn/odata/trunk/4.01%20spec/vocabularies/Org.OData.Core.V1.xml