Generate JSON archive description

[mlookaxw]

Additionally to the 'gateway.pol' and 'gateway.env' files the plugin shall generate a gateway.info.json file. The file will contain a JSON, describing the content of the archive.

Additionally the plugin will pass the content of the gateway.info.json file to the _system.artifact.info property of the buildfed tool.

The property can be used to build information endpoints which return a JSON document of the currently deployed version.