Support variables in server URLs for OAS 3.x

Created by: haitongz

Description

When running generator on an openapi 3.x document, if you use a variable for the port, generator print a warning. The warning does not block the generation, but does support openapi 3.x new feature. In addition it use default value instead in this circumstance.

Error looks like: [main] WARN o.o.codegen.utils.URLPathUtils - Not valid URL: http://example.com:{port}/v1. Default to http://localhost.

openapi-generator version

OpenAPI Generator 3.1.0

OpenAPI declaration file content or url

Command line used for generation

openapi-generator generate -i sample.yaml -g python -o /tmp/test1/

Steps to reproduce

can use yaml from: https://raw.githubusercontent.com/douglasbgray/code-gen-server-issue/master/src/main/resources/openapi.yaml

Related issues/PRs

Did not find any open issues related to this, but find related from swagger-codegen: https://github.com/swagger-api/swagger-codegen/issues/8412

Suggest a fix/enhancement

Error is coming from URLPathUtil.java. Before checking if the url can be converted into a java.net.URL, it needs to perform the variable substitution, which would resolve the {port} and generate a URL that would be valid. I find the source code URLPathUtil.java added TODO comments(TODO need a way to obtain all server URLs). I am not familiar with java. How I can finish the task myself？ thanks for your great job.