sbt-openapi-generator plugin init version

[chameleon82]

@jimschubert @wing328

As continuation of discussion OpenAPITools/openapi-generator#5406

Requested issue OpenAPITools/openapi-generator#3650

[jimschubert]

This looks awesome. I opened chameleon82#1 to make some changes to clarify package name (similar to how the Gradle plugin package is defined, matching some other main plugins like sbt-scalafmt). Also cleaned up the Copyright as we only include SmartBear sub-line on code originating from the swagger-codegen fork.

It looked like your only other question was about testing generated sources in an existing project. I'm getting my Macbook keyboard replaced tomorrow, and can look at it sometime afterward.