Improve customized loading of referenced files

[cebe]

Currently there is no way to extend the code when loading needs to be customized. Also content detection for JSON vs. YAML could be optimized.

[cebe]

php-openapi/src/spec/Reference.php

Lines 166 to 172 in 4a4892d

	$content = file_get_contents($uri);
	// TODO lazy content detection, should probably be improved
	if (strpos(ltrim($content), '{') === 0) {
	return json_decode($content, true);
	} else {
	return Yaml::parse($content);
	}