Publish UniEnc as a NuGet package

[ruccho]

• Closes Providing unienc Native Plugin as a Standalone Library #75
• Depends on Add UnboundedRecordingSession #52

Summary

• Splits Unity-dependent part of UniEnc into UniEnc.Unity
  • Readback-free encoding related APIs
  • NativeArray related APIs
• Adds pure C# csproj UniEnc into InstantReplay.Externals
  • Just refers C# source files and native binaries under Packages not to change the UPM package installation path via git
• Adds a workflow to publish NuGet package
  • Triggered by publishing release
  • Uses trusted publishing
  • NUGET_USER repository secret is required

Breaking Changes

UniEnc.VideoEncoder

• Moved to UniEnc.Unity.VideoEncoderExtensions:
  • Old: public ValueTask PushFrameAsync(Texture source, bool isGammaWorkflow, double timestamp);
  • New: public static ValueTask PushFrameAsync(this VideoEncoder encoder, Texture source, double timestamp);

UniEnc.SharedBuffer

• Type parameter is added: SharedBuffer<T> where T : unmanaged, IDisposable
• Removed: public NativeArray<byte> NativeArray { get; }
• Removed: public Span<byte> Span { get; }
• Added: public T Value { get; }

UniEnc.SharedBufferPool

• Moved to UniEnc.Unity.SharedBufferPoolExtensions:
  • Old: public unsafe bool TryAlloc(nuint size, out SharedBuffer buffer);
  • New: public static bool TryAllocAsNativeArray(this SharedBufferPool self, nuint size, out SharedBuffer<NativeArrayWrapper> buffer);