Repository app loses components metadata

Components metadata used to be unknown Maven properties, exposed as such, but since XCOMMONS-2189 they are consumed by the pom parser. In practice, it means that any XWiki repository which upgraded to XWiki 13.3+ (like extensions.xwiki.org) stopped storing and sending component metadata...

There are two things that need to be fixed:

• components metadata needs to be added to XWiki Repository protocol (XWIKI-19721)
• but for retro compatibility reasons those metadata should be put back as "unknown" properties, otherwise old instances of XWiki won't see them (this issue)