[REQ] Support incremental builds in gradle plugin

[donaldswartz]

Problem

Every time the gradle build runs, it runs the openApiGenerate task because it does not know when it is up-to-date. Gradle supports incremental builds via various methods.

Desired solution

Mark the correct Task Parameters in https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/tasks/GeneratorsTask.kt using the @Input, @InputFile, @OutputDirectory and other annotations as necessary, so that Gradle knows when all outputs are up-to-date.

More information can be found here: https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:up_to_date_checks #

[sesamzoo]

@donaldswartz, GenerateTask's parameters would need to be annotated, so the link should be https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/tasks/GenerateTask.kt

[jimschubert]

This has been contributed via #6716 and will be included in our 5.0 release.

It required bumping up from our previous minimum Gradle version to support everything fully.