Add Python "aiohttp" server generator

Description

We would like to create a Python aiohttp server generator.

Similar to how some other generators (e.g. R, Lua, PowerShell) are created, we will use the reverse engineering process to create the Lisp client generator.

• create Python aiohttp petstore server samples (implemented at least 2 endpoints to start with: getPetById, addPet)
• reverse engineer the templates and the generator (Java class)
• test the auto-generated code and revise the generator, templates

openapi-generator version

Latest master

Suggest a fix/enhancement

If anyone wants to work on this, please reply to let us know.