Improve resolving references, cache loaded files in current context

[cebe]

To avoid multiple requests to the same file, loaded files can be cached in current context.

[cebe]

php-openapi/src/spec/Reference.php

Line 107 in 4a4892d

// TODO could be a good idea to cache loaded files in current context to avoid loading the same files over and over again

[coatesap]

I have a feeling this lack of caching is the biggest performance burden when reading API files. As an experiment, I ran Speccy's resolve command over my API file first (which took ~1 sec). Using the resolved output from Speccy dropped the time of Reader::readFromYamlFile() down from 24 seconds to 10 seconds, which is fairly significant.