[API] Enriched annotation resource metadata: bookmarked URI, canonical URI, DOI, Cite-As URI & text fragments URI

[kael]

It'd be awesome if annotations objects returned by the API contained the following resource metadata in addition to the canonical URI:

• the resource URI, the one bookmarked by the user ;
• the DOI - and even other identifiers (ISBN, ISSN, etc) of the resource ;
• the Cite-As URI ;
• the text fragment URI of the highlight (there's a polyfill with interesting complex code)