Support for an UUID option as request-id

[malud]

Couper generates a unique request id with the help of https://github.com/rs/xid .
Some projects have a requirement of a real uuid which should be sent along with the request.
To provide a good log correlation, couper should be able to generate those via configuration.

The correct place for this would be the settings block within our couper configuration file.

settings {
  request_id_format = "uuid4"
}

Default should be common which would be the current behaviour with the xid package.
If the user configure the value uuid4 another generation method should be mapped to the uidFn Field of the HTTPServer struct.

Since all settings are mapped to config/runtime/HTTPConfig this one must be configurable via COUPER_REQUEST_ID_FORMAT and command flag -request-id-format.

Depends on PR #29 due to the settings block.

Pick a stable uuid package.

[cvhariharan]

Hi, I can pick this up if this is still open