Add method to provide complete OAS spec file

[ybelenko]

Package accepts deserialized parts of OAS spec as arguments right now. However, it might be much more convenient to specify path to complete OAS spec file and use only operation/model names to mock data.

As a conclusion, package should:

1. Parse yaml or json OAS spec file and store result in static variable.
2. Provide additional methods like:
   • mockModelFromSpec(string $modelName)
   • mockOperationFromSpec(string $operationId)
   • mockResponseFromSpec(string $operationId, string $responseStatusCode = 'default', string $mediaType = null).

I guess there might be issues with specs divided into many referenced files, but it's not common case and can be fixed afterwards.