Hostif trap type for Subnet routes

[rck-innovium]

Fixes #2037 

When an IP address and subnet, say 10.1.1.1/24 is on a RIF and a neighbor 10.1.1.2 is learnt, the following set of routes are needed:

1. 10.1.1.1/32 to CPU using SAI_HOSTIF_TRAP_TYPE_IP2ME,
2. 10.1.1.1/24 route : This route points to CPU when NOS wants hints to trigger ARP requests. Some NOS implementations can choose to blackhole this route. Today, SAI does not let the NOS control- whether this packets should be dropped or trapped, and more importantly when it is trapped it neither allows the user to rate-limit this traffic nor select the CPU queue.
3. 10.1.1.2/32 route: to the learnt neighbor