Add basic converter for macro plantuml

Following a discussion with rjakse, we would need to add a converter for this macro in the confluence migrator, potentially with https://github.com/xwiki-contrib/confluence/blob/master/confluence-xml/src/main/java/org/xwiki/contrib/confluence/filter/internal/macros/PlantumlRenderMacroConverter.java

There is no need to change anything when converting a plantuml macro from Confluence to XWiki. The content, the id and the parameters can stay the same.