Missing ARP and ND Suppression implementation

[msosyak]

According to https://github.com/Azure/SONiC/blob/master/doc/SONiC_202012_Release_Notes.md#evpnvxlan ARP and ND Suppression
were added in the 202012 release, but on the current codebase (master) I do not see full implementation of ARP and ND Suppression flow described in HLD https://github.com/Azure/SONiC/blob/master/doc/vxlan/EVPN/EVPN_VXLAN_HLD.md#4310-arp-and-nd-suppression. The only related code was added in #1266 but it is just a small part of all flow. Also, I found open PR with CLI command sonic-net/sonic-utilities#1970

I am going to add missed part, but want to avoid double work. So if there is some work in progress, please, let me know.

[msosyak]

@srj102 @prsunny @rpjain-nw @gord1306 Please have look and let me know if I missed something or someone already working on it.

[prsunny]

AFAIK, it is still pending. @srj102 can confirm and let you know if it can be taken up.

[gord1306]

The SAI attribute is still no conclusion and pending, the original PRs are
opencomputeproject/SAI#1026
opencomputeproject/SAI#1055