Add repository import/export capabilities

Hi,

Even though Internet access is taken for granted, I sadly work with a lot of customers that do not give any Internet access to the developers. The software must run without Internet access and they believe (wrongly in my opinion) that this can be ensured only if the developers themselves did not have Internet access while building the software.

So Nexus 2 has been a life saver for us. Whenever we need new libraries, we request them from Nexus in proxy mode in a network with Internet access. Then we export the artifacts from the file system and manually import them in the "offline" Nexus in a hosted repository and rebuild the metadata and index. We do that for maven and npm repositories.

Sadly the move to a blob store in Nexus 3 makes it impossible to replicate our work procedure if we migrate to Nexus 3.

I'd like if you could add a repository export/import feature in Nexus 3 like in Artifactory, where an import would merge the new content into an existing repository. Also a filter on the export by date would be great.

Regards