[Ada] Add Ada support for client code generator

[stcarrez]

Description

Swagger does not support Ada language. There is some interest in the Ada community
to use and implement REST clients.

Swagger-codegen version

Latest master

Suggest a fix/enhancement

I've started to add Ada support for Swagger codegen. I've forked the project and I'm adding
Ada client code generator. The code generator is now able to generate the Ada client side.
The generated Ada code uses a small Ada library that I'm building as a separate project
(see https://github.com/stcarrez/swagger-ada).

I've been in touch with William and I plan to make a pull request for this PR shortly.
For now, the changes are available in https://github.com/stcarrez/swagger-codegen