Enhance IPSecSAParams in tunneloffload.proto with 'salt' parameter

[jpulloka]

"salt" is a four octet value assigned at the time of security association creation. Currently tunnel offload proto is not exposing any attribute to set 'salt' value for IPSec SA. I think, we need to enhance IPSecSAParams structure with "salt" attribute so that application can explicitly make use of this attribute to configure salt value.