Add support for OpenAPI file split into multiple files

[xserrat]

Hi, I'm wondering if is there any possibility to introduce support for reading multiple files of a split OpenAPI file as the specification supports:

https://swagger.io/docs/specification/using-ref/#allowed-places

Now the validator throws an error:

"Invalid ref: paths\/endpoint.yaml"

The expected behaviour would be to read all files based on the $ref field where the file path is specified.

Thank you in advance!