Support for signed references in Blob

As part of JCR-3534 Jackrabbit has introduced support for signed references for securely transferring binary references across repositories sharing the DataStore

Support for FileDataStore was added as part of OAK-805. However support for signed references is missing.

As a fix signed references should be supported

1. org.apache.jackrabbit.oak.plugins.blob.BlobStoreBlob#getReference - It should probably delegate to org.apache.jackrabbit.core.data.AbstractDataRecord#getReference if the underlying BlobStore is based on DataStore
2. It should be possible to specify the signing key using OSGi config