[202211]Custom monitoring based priority tunnels

[siqbal1986]

This PR implements the Vxlan ECMP priority tunnels where route next hop groups are switched based on the state of the next hop monitor session and their priority. This implementation allows primary and secondary next-hop groups which are switched based on the monitor state of the next hops. Please see the included tests to see what this code would do for detail.

cherry-pick https://github.com/sonic-net/sonic-swss/pull/2719

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