allow for oatpp::swagger::Resources to be inherited and extended

[ygerlach]

I would like to embed all resources into my oatpp binary, so it can be updated by just replacing one file and reduce any dependencies that the binary has to its environment.
For my own resource files i have already implemented a little build tool to do this. Now i want to embed the swagger resources as well. But i cant, because i dont have a real folder with the content inside. And i can not construct oatpp::swagger::Resources without such a folder.

So my change proposed here allows me to inherit oatpp::swagger::Resources (using the protected ctor) and implement the logic to fetch internal resources my self.
I also made the hasEnding method static and fixed the argument in loadResources.