Add IBufferWriter<byte> overload to NonGeneric API

[neuecc]

I'm trying to support BodyWriter in netcoreapp3.0 to AspNetCoreMvcFormatter.
https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/request-response?view=aspnetcore-3.1

This code will become following.

#if NETSTANDARD2_0
return MessagePackSerializer.SerializeAsync(context.ObjectType, context.HttpContext.Response.Body, context.Object, this.options, context.HttpContext.RequestAborted);
#else
await MessagePackSerializer.SerializeAsync(context.ObjectType, context.HttpContext.Response.BodyWriter, context.Object, this.options, context.HttpContext.RequestAborted);
#endif

But currently NonGeneric API does not covered full Generic APIs so cannot implement it.

[AArnott]

There is a MessagePackWriter overload in the non-generic API, FWIW. So you can make it work today simply by wrapping the IBufferWriter<T> in one of those first.