Switch to PSR-17 and PSR-18 for HTTP interface

See https://www.php-fig.org/psr/psr-17/ and https://www.php-fig.org/psr/psr-18/.

Related: https://github.com/KnpLabs/php-github-api/pull/885, https://github.com/KnpLabs/php-github-api/pull/888