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.