MINOR: net_helper: add sample converters to decode ethernet frames

MINOR: net_helper: add sample converters to decode ethernet frames

This adds a few converters that help decode parts of ethernet frame
headers:
  - eth.data : returns the next header (typically IP)
  - eth.dst  : returns the destination MAC address
  - eth.hdr  : returns only the ethernet header
  - eth.proto: returns the ethernet proto
  - eth.src  : returns the source MAC address
  - eth.vlan : returns the VLAN ID when present

These can be used with the tcp-ss bind option. The doc was updated
accordingly.