[Eiffel] Enhancements to the Eiffel generator

Description

Here is a list of enhancements I can think of.

• Create shell scripts, batch files to generate Eiffel Petstore samples
• Setup CI (travis, shippable, etc) to cover the Eiffel Petstore samples.
• Refactor part of the code into an Abstract Eiffel base class, which can be later used in Eiffel server stub generators
• Add Eiffel Style guide to the contribution guidelines so that Eiffel contributors know what to follow in terms of code style
• Add {{#required}} .. {{/required}} to make sure the parameters (query, form, header, body) is non-null (exmaple)
• Add support for validation rules defined in the spec (e.g. validation for parameters in Ruby API layer)

Please feel free to reply with other suggestions.

Swagger-codegen version

Latest master

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/pull/6036

Suggest a Fix/Enhancement

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

cc @jvelilla