rpc: Add support for resolver options callback

This adds support for a callback returning name resolver options. This
is required for powercycling offline nodes. While it would be possible
to implement this using a separate RPC client class, doing so would
require some code duplication with the standard “RpcRunner” class.

The callback can generate the options based on the call arguments.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>