Description of problem: OVN gateway routers support "ecmp_symmetric_reply" behavior for ecmp static routes since v20.09.0 (https://github.com/ovn-org/ovn/commit/4fdca656857d4a5caeec35ae813888cb9e403e5e) However, the routing decision can be overridden through re-route logical router policies. These, in turn, support specifying multiple next-hops (ECMP) since v21.03.0 (https://github.com/ovn-org/ovn/commit/35b00c7e79900787e510bb203c5cddfb895f6c4f). It's desirable that router policies also support the "ecmp_symmetric_reply" semantics, in order to allow reply traffic to always be sent out on the same path on which the initiating traffic was received. This behavior should be on-demand, opt-in, configurable through a new Logical_Router_Policy option, e.g. "ecmp_symmetric_reply".