[vnetorch] Add ECMP support for vnet tunnel routes (sonic-net#1960)

What I did
Add functions to create/remove next hop groups for vnet tunnel routes.
Count the reference count of next hop groups to create and remove as needed.
Share the counter of next hop groups with routeorch.
Add vs test

Why I did it
To add support for overlay ECMP.

How I verified it
Verify ECMP groups are properly created and removed with the functions.
Verify vs test passes

orchagent/nexthopkey.h

@@ -68,6 +68,8 @@ struct NextHopKey
         mac_address = keys[3];
     }
 
+    NextHopKey(const IpAddress &ip, const MacAddress &mac, const uint32_t &vni, bool overlay_nh) : ip_address(ip), alias(""), vni(vni), mac_address(mac){}
+
     const std::string to_string() const
     {
         return ip_address.to_string() + NH_DELIMITER + alias;

orchagent/routeorch.cpp

@@ -2186,3 +2186,17 @@ bool RouteOrch::removeOverlayNextHops(sai_object_id_t vrf_id, const NextHopGroup
     return true;
 }
 
+void RouteOrch::increaseNextHopGroupCount()
+{
+    m_nextHopGroupCount ++;
+}
+
+void RouteOrch::decreaseNextHopGroupCount()
+{
+    m_nextHopGroupCount --;
+}
+
+bool RouteOrch::checkNextHopGroupCount()
+{
+    return m_nextHopGroupCount < m_maxNextHopGroupCount;
+}

orchagent/routeorch.h

@@ -137,6 +137,10 @@ class RouteOrch : public Orch, public Subject
     bool createFineGrainedNextHopGroup(sai_object_id_t &next_hop_group_id, vector<sai_attribute_t> &nhg_attrs);
     bool removeFineGrainedNextHopGroup(sai_object_id_t &next_hop_group_id);
 
+    void increaseNextHopGroupCount();
+    void decreaseNextHopGroupCount();
+    bool checkNextHopGroupCount();
+
 private:
     SwitchOrch *m_switchOrch;
     NeighOrch *m_neighOrch;