Add spec for L3 Agent Scheduler API support in ML2/OVN

This spec proposes adopting the existing L3 Agent Scheduler API (l3_agent_scheduler extension) for the ML2/OVN mechanism driver, enabling operators to query router-to-gateway-chassis associations through the standard Neutron API.

A new API extension (l3_agent_scheduler_ha_priority) is proposed to expose the HA_Chassis priority field, which conveys the failover ordering within an HA_Chassis_Group – information not present in the current API.