Add IProtobufDecoder extension point for custom gRPC decoding

[haga-rak]

Summary

• Adds IProtobufDecoder interface and ProtobufDecodeContext struct as public API for custom protobuf message decoding
• Adds FormatSettings.ProtobufDecoder property to wire in a custom decoder
• Includes ProtobufDecoder.Create(Func<...>) factory for delegate-based usage
• Custom decoder is tried first, with protoc-based and raw wire-format decoding as fallbacks

This lets library consumers provide their own decoding logic (e.g., via protobuf-net.Reflection) without requiring the protoc CLI on PATH.

Test plan

• Verify dotnet build src/Fluxzy.Core succeeds
• Verify existing gRPC formatting tests pass with no decoder set (null by default)
• Verify that when a custom IProtobufDecoder is set on FormatSettings, it is called before protoc-based decoding
• Verify that returning null from IProtobufDecoder.TryDecode falls through to protoc-based and then raw decoding
• Verify ProtobufDecoder.Create(func) correctly wraps a delegate