Add the possibility to map the Confluence group 'anonymous-users' with an XWiki user or group

Currently the Confluence group "anonymous-users" is mapped with the XWiki Guest user (see https://github.com/xwiki-contrib/confluence/blob/76cdc8b62a1d8d517b37cda58fcb12473d697e0a/confluence-xml/src/main/java/org/xwiki/contrib/confluence/filter/internal/input/ConfluenceInputFilterStream.java#L764) and there is no way to map the group with another XWiki user or group.

The objective of this ticket is to support mapping the "anonymous-users" confluence group with another XWiki user or group if it is mentioned in the group mapping field otherwise the group should be mapped with the Guest user as it is currently.