Return odata.etag in collections also for odata=minimalmetadata

For single entity responses the ETag is contained in the ETag response header.

For multi-entity responses the header is not sufficient, instead clients need to receive the ETag in the entity metadata, i.e. the odata.etag annotation.

We do have a Core annotation that tells clients which entity properties influence the ETag, but we currently have no way of telling the client which algorithm the server uses to calculate the ETag.