Read $remote_addr attribute for AccessLogHandler (trusted proxies)

[clue]

This changeset adds support for trusted proxies by reading the $remote_addr attribute for the AccessLogHandler like this:

<?php

require __DIR__ . '/../vendor/autoload.php';

class TrustedProxyMiddleware
{
    public function __invoke(Psr\Http\Message\ServerRequestInterface $request, callable $next)
    {
        // use 127.0.0.1 as trusted proxy to read from X-Forwarded-For (XFF)
        $remote_addr = $request->getAttribute('remote_addr') ?? $request->getServerParams()['REMOTE_ADDR'] ?? null;
        if ($remote_addr === '127.0.0.1' && $request->hasHeader('X-Forwarded-For')) {
            $remote_addr = preg_replace('/,.*/', '', $request->getHeaderLine('X-Forwarded-For'));
            $request = $request->withAttribute('remote_addr', $remote_addr);
        }
        return $next($request);
    }
}

$app = new FrameworkX\App(
    TrustedProxyMiddleware::class,
    FrameworkX\AccessLogHandler::class,
    FrameworkX\ErrorHandler::class
);

// Register routes here, see routing…

$app->run();

This is the next step in adding more options to control access logging in follow-up PRs as discussed in #169 and #172.

Builds on top of #174 and #175