MINOR: log: support "raw" logformat node typecast

MINOR: log: support "raw" logformat node typecast

"raw" logformat node typecast is a special value (unlike str,bool,int..)
which tells haproxy to completely ignore logformat options (including
encoding ones) and force binary output for the current node only. It is
mainly intended for use with JSON or CBOR encoders in order to generate
nested CBOR or nested JSON by storing intermediate log-formats within
variables and assembling the final object in the parent log-format.

Example:

  http-request set-var-fmt(txn.intermediate) "%{+json}o %(lower)[str(value)]"

  log-format "%{+json}o %(upper)[str(value)] %(intermediate:raw)[var(txn.intermediate)]"

Would produce:

   {"upper": "value", "intermediate": {"lower": "value"}}