Support relative server URL

Created by: cbornet

Description

Generator doesn't support relative URL for OAIv3 servers. Whereas the spec specifies

This URL supports Server Variables and MAY be relative, to indicate that the host location is relative to the location where the OpenAPI document is being served.

openapi-generator version

3.0.1

OpenAPI declaration file content or url

OpenAPI petstore's server URL is relative and it's /v3

Steps to reproduce

• Start the docker petstore on port 8080
• Generate the spring-cloud client
• The default URL in PetApiClient should be http://localhost:8080/v3 but it's http://localhost/v3