tools: Add dependecy download helpers

This patch adds 2 helpers to download dependencies for compiler and
stub generator during their execution.

They take list of external dependencies as a parameter and directory
where they must be located. Helpers check if these dependencies exist
in the given directory and download not existing ones.

settings.gradle

@@ -7,3 +7,4 @@ include ':backend.native'
 include ':runtime'
 include ':common'
 include ':backend.native:tests'
+include ':tools:helpers'