Refactor transfer API and open it to outside

[thekid]

This way, we can use it to implement signed requests as seen here: https://docs.aws.amazon.com/general/latest/gr/create-signed-request.html:

<?php

use io\streams\MemoryOutputStream;
use peer\http\HttpRequest;
use util\Secret;
use webservices\rest\io\Transfer;

/**
 * Signing AWS API requests
 *
 * @see  https://docs.aws.amazon.com/general/latest/gr/create-signed-request.html
 */
class AwsSignedRequests extends Transfer {
  const HASH= 'sha256';
  const ALGO= 'AWS4-HMAC-SHA256';

  private $service, $accessKey, $secretKey, $session, $region, $userAgent;

  public function __construct($service, $accessKey, Secret $secretKey, $session= null, $region= '*') {
    $this->service= $service;
    $this->accessKey= $accessKey;
    $this->secretKey= $secretKey;
    $this->session= $session;
    $this->region= $region;
    $this->userAgent= 'xp-aws/1.0.0 OS/'.php_uname('s').'/'.php_uname('r').' lang/php/'.PHP_VERSION;
  }

  /** Returns signature headers */
  public function signature(HttpRequest $request, string $payload, int $time= null): array {

    // Implementation stripped for brevity reasons!

    return $headers;
  }

  /**
   * Start buffering.
   *
   * @param  peer.http.HttpConnection $conn
   * @param  peer.http.HttpRequest $request
   * @return io.streams.OutputStream
   */
  public function start($conn, $request) {
    return new MemoryOutputStream();
  }

  /**
   * Finish buffering, sign request, then send it.
   *
   * @param  peer.http.HttpConnection $conn
   * @param  peer.http.HttpRequest $request
   * @param  io.streams.OutputStream $output
   * @return peer.http.HttpResponse
   */
  public function finish($conn, $request, $output) {
    if (null === $output) {
      $request->addHeaders($this->signature($request, ''));
      return $conn->send($request);
    }
 
    $bytes= $output->bytes();
    $request->setHeader('Content-Length', strlen($bytes));
    $request->addHeaders($this->signature($request, $bytes));

    $stream= $conn->open($request);
    $stream->write($bytes);
    return $conn->finish($stream);
  }
}