EBookBackend: Added apis for backends to support Direct Read Access

EBookBackend: Added apis for backends to support Direct Read Access

e_book_backend_get_direct_book:
  A backend can return an EDataBookDirect from here to indicate
  that it supports direct read access, the properties of the EDataBookDirect
  are used by the client to access the book directly

e_book_backend_configure_direct:
  A backend opened in direct read access mode can be configured with
  the string that it's server side counterpart provided, this string
  belongs to the backend; for the local file backend it is used as
  a path to ensure the same SQLite DB is opened by the client as the
  one running in the server.