MINOR: filters/lua: Add request and response HTTP messages in the lua TXN

MINOR: filters/lua: Add request and response HTTP messages in the lua TXN

When a lua TXN is created from a filter context, the request and the response
HTTP message objects are accessible from ".http_req" and ".http_res" fields. For
an HTTP proxy, these objects are always defined. Otherwise, for a TCP proxy, no
object is created and nil is used instead. From any other context (action or
sample fetch), these fields don't exist.