Addition of AVAILABLE_OPTIONS system property for language selection

Description

Currently only hiding languages are available to us. With each release there is the potential for new languages that we may potentially want to hide for a number of reasons. As this is the case, we're unable to automatically accept a release version into our stack. Currently we verify what languages are available, update the HIDDEN_OPTIONS property and then push that release into our stack.

By allowing for AVAILABLE_OPTIONS, we would potentially be able to automate that subsequent push (on image release). If we could define only the languages we want to make available, we remove the possibility we may show languages we want to hide for some reason.

If a language was defined as both HIDDEN and AVAILABLE, HIDDEN should take precedence.

Swagger-codegen version

3.0.11

Swagger declaration file content or url

N/A

Command line used for generation

N/A

Steps to reproduce

N/A

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/pull/9686

Suggest a fix/enhancement

N/A

Review Swagger Codegen's language-selection and HIDDEN_OPTIONS handling, then compare the proposed behavior with related PR #9686. Define AVAILABLE_OPTIONS so it limits exposed languages while HIDDEN_OPTIONS takes precedence, and verify the image-release automation scenario described in the issue.