Implement JsonSerializable so OpenApi instance can be serialized using json_encode()

[grimmdude]

Allows the ability to serialize OpenApi instance like this:

$openApi = Reader::readFromYaml($yaml);
$json = json_encode($openApi);

[cebe]

Looks useful, but I'd add some tests before merging.

[grimmdude]

Looks useful, but I'd add some tests before merging.

Hi @cebe, I'll leave the tests to you. It just calls the existing getSerializableData() method when using json_encode().