feat: make HTTP header to log attribute mapping configurable

[stuartwdouglas]

Replace the hard-coded X-Blox-Workstation-Id header extraction in LoggingMiddleware with a general-purpose header-attrs config map in the log block.

Any header can now be mapped to a log attribute via config:

log {
  header-attrs = {
    "X-Blox-Workstation-Id" = "blox.workstation_id"
    "X-Request-Id"          = "request_id"
  }
}

Changes

• Added HeaderAttrs map[string]string to logging.Config
• Updated LoggingMiddleware to accept the map and iterate over it
• Threaded the config through newServer

[alecthomas]

#256 won the race! But also consolidates the middleware into the logging package