Add a nuxeo.conf property to expose the final Blob URL

If a document's blob provider is configured for direct download, it's poss ible to get direct links to the final download URL (to S3 or CloudFront typically) returned in the JSON document output by activating a Configuration Service property:

  <require>org.nuxeo.ecm.core.io.download.DownloadService</require>
  <extension target="org.nuxeo.runtime.ConfigurationService" point="configuration">
    <property name="org.nuxeo.download.url.follow.redirect">true</property>
  </extension>

We want to be able to configure this configuration property through a nuxeo.conf property:

org.nuxeo.download.url.follow.redirect=true