Issue #2497691 by Crell, cilefen, marvin_B8, dawehner: Include Symfony PSR-7 bridge library

core/composer.json

core/composer.lock

core/core.services.yml

core/vendor/composer/autoload_psr4.php

core/vendor/composer/installed.json

core/vendor/psr/http-message/LICENSE

core/vendor/psr/http-message/README.md

core/vendor/psr/http-message/composer.json

core/vendor/psr/http-message/src/MessageInterface.php

core/vendor/psr/http-message/src/RequestInterface.php

core/vendor/psr/http-message/src/ResponseInterface.php

core/vendor/psr/http-message/src/ServerRequestInterface.php

core/vendor/psr/http-message/src/StreamInterface.php

core/vendor/psr/http-message/src/UploadedFileInterface.php

core/vendor/psr/http-message/src/UriInterface.php

core/vendor/symfony/psr-http-message-bridge/.travis.yml

core/vendor/symfony/psr-http-message-bridge/CHANGELOG.md

...fony/psr-http-message-bridge/Factory/DiactorosFactory.php

...psr-http-message-bridge/Factory/HttpFoundationFactory.php

...sr-http-message-bridge/HttpFoundationFactoryInterface.php