Add router_factory to l3-agent and L3 extension API

Currently, most implementations override the L3NatAgent class itself
for their own logic since there is no proper interface to extend
RouterInfo class. This adds unnecessary complexity for developers
who just want to extend router mechanism instead of whole RPC.

This spec introduces RouterFactory class which acts on factory for creating
router_info class, and add new l3 extension API which enable to dynamically
add router_Info to the factory. Now plugin developers can use new extension
API for their own specific router.