table: support selector table

table: support selector table

A selector table is made up of groups of weighted members, with a
given member potentially part of several groups. The select operation
returns a member ID by first selecting a group based on an input group
ID and then selecting a member within that group based on hashing one
or several input header/meta-data fields. It is very useful for
implementing an ECMP/WCMP-enabled FIB or a load balancer. It is part
of the action selector described by the P4 Portable Switch
Architecture (PSA) specification.

Signed-off-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>