Add "servers" support to generators

[wing328]

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 #1280. We want to do the same to other generators.

openapi-generator version

Latest master

Related issues/PRs

#1280
#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.

[ybelenko]

Is this relevant to client generators only? Can't imagine any use case when servers values are useful from backend. We can list available servers in README of server stubs and that's all, imho.