fix: [PubSub] Prevent fatal error when using emulator without grpc

[kynx]

This fixes https://github.com/googleapis/google-cloud-php/issues/7187

Unfortunately using the PubSub emulator is still not entirely straightforward with REST: the https schema is hardcoded in Google\ApiCore\RequestBuilder::buildUri(), probably for very good reason!

I have managed to get it to work by supplying a modified httpHandler:

// double the # of characters before truncation by default
        $bodySummarizer = new BodySummarizer(240);
        $stack = HandlerStack::create();
        $stack->remove('http_errors');
        $stack->unshift(Middleware::httpErrors($bodySummarizer), 'http_errors');
        $stack->push(Middleware::mapRequest(static function (RequestInterface $request) {
            $uri = $request->getUri();
            return $request->withUri($uri->withScheme('http')); // convert https -> http for emulator
        }));
        $client = new Client(['handler' => $stack]);

        $pubSubClient = new PubSubClient([
            'projectId'         => 'emulator-project',
            'credentialsConfig' => [
                'keyFile' => [
                    "client_id"     => "fake-fake-fake.apps.googleusercontent.com",
                    "client_secret" => "fake-fake-fake",
                    "refresh_token" => "fake-fake-fake",
                    "type"          => "authorized_user",
                ],
            ],
            'transportConfig' => [
                'rest' => [
                    'httpHandler' => [HttpHandlerFactory::build($client), 'async'],
                ],
            ],
        ]);

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[kynx]

@bshaffer Thanks for adding this to the next release! Sorry about the CS: CONTRIBUTING.md seems a bit out of data - the composer style script mentioned there no longer exists.

I'll probably forget everything about this within a month. If I wanted to add the example above to the online docs, where would I make a PR?