[REQ] Switch Gradle plugin to use the File Provider API for Inputs/Outputs

Created by: mcwhitak

Right now the Gradle plugin relies on all of it's Input and Output properties as Strings, even those that are representing file paths.

While users can convert from properties of Files -> String and Strings -> Files it can be very cumbersome, and doesn't lend itself well with connecting the inputs/outputs of the Generate task with other tasks.

This feature request is to change the GenerateTask.kt file to use the following properties instead of Property<String> for file based input/outputs.

RegularFileProperty DirectoryProperty