* [gradle-plugin] Initial commit

* Clarify comments on file constraints

When a user sets the models, apis, or supportingFiles environment
variables, any one of these being set disables generation for the other
two.  This could be confusing to users, so I've added some clarification
text in the comments for these properties.

In addition, I've cleaned up the extension on
Property.ifNotEmpty, to avoid using Suppress annotations where it's not
necessary. The change creates a local variable of type T?, allowing
Kotlin to track the variable's nullable state at compile time.

* Move gradle plugin under modules

* Move kt files under kotlin source set. Add sample.

* [gradle] map-like options as maps

* Add tests for gradle validate task

* Apply gradle plugin to mvn install phase

* [gradle] Testing remaining gradle tasks

* Add gradle plugin to the integration doc

* Update gradle plugin README with task options

* Gradle readme formatting