Make feed plugin create feed entries in syntax 2.0 by default + conf options to be backward compatible or use a different syntax

Currently feed plugin creates feed entry in syntax 1.0 with hardcoded content :

#includeForm('XWiki.FeedEntryClassSheet')

In the effort to have most content by default in syntax 2.0 (for consistency, better support, better WYSIWYG, annotations, etc.), the feed plugin should create entries in this syntax, with options to support :
1- Backward compatilibity (for example for users of XWiki Watch that would want to upgrade their wiki and have entries still created in syntax 1.0)
2- Other syntaxes / other content.

Thus I propose to introduce 2 configrations options of the feed plugin :

• xwiki.plugins.feed.entryContent (defaults on

  {{include document="XWiki.FeedEntryClassSheet" /}

• xwiki.plugins.feed.entrySyntaxId (defaults on "xwiki/2.0")