Added DATA opcode support to ApfGenerator

This commit adds support for the "DATA" opcode to ApfGenerator. This
opcode add data section to the beginning of the program which can be
used to construct the offload packet later.

* design doc: go/apf-v6-proposal

Bug: 293811969
Test: TH
Change-Id: I3c05538b7454259d8a80911b075c811ea7403d67
2 files changed