Add `files` command to server mode

[sim642]

Currently the json-messages output used by GobPie contains both the messages themselves and the analysis dependency files.
Instead of going through a JSON file, GobPie would retrieve the messages using the messages command in server mode. But no counterpart currently exists for retrieving the files in server mode.

This isn't currently strictly necessary as GobPie isn't using the dependency files list anyway, but at some point it might be.