Add allows ref struct constraint to TBufferWriter for .NET 9+

[juho-hanhimaki]

Resolves #447

Overview
This PR adds the C# 13 allows ref struct anti-constraint to the TBufferWriter generic parameter across MemoryPackWriter and related core interfaces for .NET 9+. This enables users to implement custom buffer writers as ref structs, unlocking zero-allocation serialization for native and unmanaged memory arenas.

The Problem Solved
Implementing IBufferWriter<byte> normally forces a heap allocation for a MemoryManager<T> because GetMemory() must return a Memory<byte>. Since MemoryPackWriter strictly uses GetSpan(), this allocation is wasted. By allowing a ref struct buffer writer, consumers can skip the heap allocation entirely and throw NotSupportedException in their GetMemory() implementation.

Implementation Details & CS9050 Workaround
During implementation, updating MemoryPackWriter<TBufferWriter> to store the TBufferWriter as a ref field results in compiler error CS9050: A ref field cannot refer to a ref struct. Even though MemoryPackWriter itself is a ref struct, C# currently forbids declaring a ref T field if T is an anti-constraint.

To bypass this safely while maintaining zero-allocation performance, the internal field was mapped to a ref byte:

#if NET9_0_OR_GREATER
    ref byte bufferWriterRef;
    ref TBufferWriter bufferWriter => ref Unsafe.As<byte, TBufferWriter>(ref bufferWriterRef);
#endif

Because MemoryPackWriter is a ref struct, the GC's tracking of ref byte is identical to tracking ref TBufferWriter. The memory is guaranteed not to escape the stack or pinned context.

Breaking Changes

• Older Frameworks: This change is wrapped in #if NET9_0_OR_GREATER compilation directives. Existing .NET 7 and .NET 8 users are completely unaffected.
• .NET 9 Users: This introduces a source-breaking change for custom formatters compiled against .NET 9. Anyone who has written a custom formatter by inheriting from MemoryPackFormatter<T> or using [MemoryPackOnSerializing] / [MemoryPackOnSerialized] with a TBufferWriter parameter will need to add the allows ref struct constraint to their method signatures to match the updated base class/interface.
• Source Generator: The MemoryPack Source Generator does not require updates. It emits explicit interface implementations (e.g., static void IMemoryPackable<T>.Serialize<TBufferWriter>(...)), which implicitly inherit the constraints from the interface.