Support for Fine-Grained ECMP (consistent hashing) with dynamic nexthops

[ashutosh-agrawal]

Currently, Fine-Grained ECMP feature in SONiC requires the list of nexthops to be explicitly specified through configuration. This feature enhancement will add support for dynamically discovering nexthops from the route updates. This will bring SONiC's fine-grained hashing feature more in-line with consistent or resilient hashing supported by other NOSes.