Introduce safe property access for Unity NDK and export paths

[teogor]

Summary:

This PR adds an internal API (@InternalDrifterApi) to provide safer and more flexible access to Drifter properties from Gradle build scripts. It introduces the getSafeDrifterUnityPathNdk() and getSafeDrifterUnityPathExport() functions to ensure required properties are present and handle errors gracefully.

Motivation:

• Prevent potential crashes due to missing properties.
• Improve error handling and provide informative messages.
• Allow for future changes to property handling without breaking external code.

Usage:

android {
  unityOptions(
    ndkPath = getSafeDrifterUnityPathNdk()
    exportFolder = getSafeDrifterUnityPathExport()
  )
}