Propagate/expose object headers from storage adapters

[nukemberg]

In order to facilitate caches and to set file metadata/headers on http server and nfs/dav/fuse mounts - we need to propagate object headers from storage fetch method.

• Storage adapter fetch should return a struct with readercloser reference, and object metadata
• CDR metadata needs to be extended

[nukemberg]

@ozkatz we can put all them headers in a map trash can or we can create a struct with important ones (e.g. ETAG, expire, timestamp). or both. preference?