Improve atomicfu gradle extension

[mvicsokolova]

• Support IR transformations flags in atomicfu {} gradle extension and get rid of flags in gradle.properties (kotlinx.atomicfu.enableJvmIrTransformation = true etc)
• The user should be able to turn off transformations for the specified backend completely and use the library as a runtime dependency
• Make sure the new extension works for both Groovy and Kotlin DSL: Project.atomicfu extension is not available for build.gradle.kts #357
• Here is another related issue: dependenciesVersion is null by default in AtomicFUPluginExtension #137
• Take into consideration: Support control over enabling plugin transformations between test and main source sets #343

[mvicsokolova]

The issue with atomicfu {} gradle extension: #357