Add experimental Unity 6000 implementation for Android.

[timbotimbo]

This PR merges into the branch experimental/unity_6000 to allow people to test this without affecting the main plugin in master.

Description

Unity 2023.3 turned into 6000 preview, which will eventually become the next LTS version.
For ios and web you will likely be able to build using the existing 2022.x plugin.
However Android had some breaking changes in Unity 2023.1.

This PR is a base implementation for supporting Unity 6000 on Android.
For now this is in a separate branch from master for testing.

Fixed changes in the Unity 6000 android export

• Unity 2023+ makes the UnityPlayer class abstract and exposes UnityPlayerForServiceOrActivity instead.
  UnityPlayer no longer extends FrameLayout which breaks some overrides.
• The exported UnityLibrary/build.gradle now imports a gradle file from a .../shared folder.
• Gradle versions are updated to 8.x
• Java is updated to 17

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore