pipeline: add header look-ahead instruction

pipeline: add header look-ahead instruction

Added look-ahead instruction to read a header from the input packet
without advancing the extraction pointer. This is typically used in
correlation with the special extract instruction to extract variable
size headers from the input packet: the first few header fields are
read without advancing the extraction pointer, just enough to detect
the actual length of the header (e.g. IPv4 IHL field); then the full
header is extracted.

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