Use relative path to oas-3.0.0.json

[rozmansi]

I am running Oat++ server behind a reverse proxy. The public URL is like https://www.example.com/myservice/ while internally it gets the content from my Oat++ server at http://host.local:8080/.
Mind the external path /myservice/swagger/ui maps to internal path /swagger/ui.

Now, the Swagger UI is using absolute path to load OAS from /api-docs/oas-3.0.0.json. And this path is hardcoded. This makes it fail to load when the external root is different from the Oat++ server one.

I suggest like loading CSS using relative path, oas-3.0.0.json could also be changed to load using a relative path like ../api-docs/oas-3.0.0.json. This should allow client to load the right OAS regardless the path the service is exported to the public.

This might be related to #90. Solving this would also solve #77.

[rozmansi]

Found out, I can modify the OAS path in "Explore" field in Swagger UI header. If I change it manually to ../api-docs/oas-3.0.0.json, it loads correctly.

Only to find out that Swagger nevertheless thinks the base URL is https://www.example.com/ instead of https://www.example.com/myservice/. This produces wrong URLs for all examples.

So, solving OAS path would have been just a start.