[ruby] Support aliasing of API keys

[zippolyte]

Adds support for API key aliasing as it's done in Go, Java and python:

• [go-experimental] Support aliasing of API keys #4940
• [java] Support aliasing of API keys #4966
• [feature][python] Support aliasing of API keys #6469

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[zippolyte]

cc @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[zippolyte]

cc @wing328@jimschubert, this is very similar to the other PRs you reviewed, please have a look 🙂 🙇

[wing328]

Restarted the failed CI job. Let's see how that goes.

[wing328]

Travis CI tests passed via https://travis-ci.org/github/OpenAPITools/openapi-generator/builds/752969100

[zippolyte]

Thanks for the merge 🙇 !