Add pluggable report converter
unoconv never shows to be a reliable system and so we should not encourage its usage. More over, it depends on libreoffice which adds a lot of bloat to the installation.
Indeed the Report engine should have a mechanism to register (by configuration) converter with a list of input format and output format supported. The Report engine will be responsible to select the best converter depending of the input format. The ir.action.report should show as options only available format.