Access BlobHolder secondary blobs through rendition API

We want to provide an access to the document attached files trough the CMIS API. The "rendition" extension is providing a read only access to multiple contents. This is what we want to implement.

Impacts are

• org.nuxeo.ecm.core.opencmis.impl.server.NuxeoObjectData.getRenditions(DocumentModel, CallContext) -> should build and return rendition data from the document blob holder
• insure that the main content is excluded from the returned renditions data