Add "servers" support to generators

Description

servers in OAS v3 allows users to define mutiple base URL with variable support. We've added servers support to the Ruby generator via https://github.com/OpenAPITools/openapi-generator/pull/1280. We want to do the same to other generators.

openapi-generator version

Latest master

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/1280 https://github.com/OpenAPITools/openapi-generator/issues/590

Suggest a fix/enhancement

If anyone wants to help on this task, please reply to let us know which generator(s) you want to work on.