Store Kotlin/Native version in properties

Previously the Kotlin/Native version was hardcoded in sources of the
Gradle plugin. Such an approach is inconvenient when we want to
build Kotlin with a custom K/N version but without changes in sources
(e.g. during CI daily runs).

This patch adds a project property `versions.kotlin-native` which can
be set during build to override Kotlin/Native version. Also this
patch gets rid of hardcoding this version in sources and stores it in
a properties file packed with the Gradle plugin in a jar.

buildSrc/src/main/kotlin/dependencies.kt

@@ -96,6 +96,8 @@ val Project.protobufRepo: String
 fun Project.protobufLite(): String = "org.jetbrains.kotlin:protobuf-lite:$protobufVersion"
 fun Project.protobufFull(): String = "org.jetbrains.kotlin:protobuf-relocated:$protobufVersion"
 
+val Project.kotlinNativeVersion: String get() = property("versions.kotlin-native") as String
+
 fun File.matchMaybeVersionedArtifact(baseName: String) = name.matches(baseName.toMaybeVersionedJarRegex())
 
 private val wildcardsRe = """[^*?]+|(\*)|(\?)""".toRegex()