Re-implement plugin using Kotlin and Gradle incremental build APIs

[adam-dpg]

Work in progress!

You can already use this release - see the README here

Depends on https://github.com/snyk/gradle-plugin/pull/16

• Configure the extension and task properties to use task-avoidance https://docs.gradle.org/current/userguide/more_about_tasks.html#sec:up_to_date_checks
• Expose more properties so users can configure the plugin more easily
• Use Property<>s and Provider<>s to make the plugin compatible with the Gradle build and configuration caches https://docs.gradle.org/current/userguide/configuration_cache.html
• Validates the downloaded Snyk CLI checksum

Resolves https://github.com/snyk/gradle-plugin/issues/4 Resolves https://github.com/snyk/gradle-plugin/issues/5 Resolves https://github.com/snyk/gradle-plugin/issues/6 Resolves https://github.com/snyk/gradle-plugin/issues/7 Resolves https://github.com/snyk/gradle-plugin/issues/8 Resolves https://github.com/snyk/gradle-plugin/issues/11 Resolves https://github.com/snyk/gradle-plugin/issues/13 Resolves https://github.com/snyk/gradle-plugin/issues/14

TODO

• [ ] up-to-date check on the CLI version
• [ ] Tests
• [ ] Add KDoc to extension properties
• [ ] Add KDoc to task properties