Support `struct` types

[ChristophLindemann]

Currently struct and readonly struct can not be serialized/deserialized without writing a specific converter.

The ObjectConverter class is constrained to where T : class. So even if you write you own generic struct converter, deserialization does not work with the DiscriminatorConvention as it expects to get an IObjectConverter<T>:

Dahomey.Cbor/src/Dahomey.Cbor/Serialization/Converters/ObjectConverter.cs

Lines 277 to 290 in 527b72f

	if (_discriminatorConvention != null)
	{
	CborReaderBookmark bookmark = reader.GetBookmark();
	if (FindItem(ref reader, _discriminatorConvention.MemberName))
	{
	// discriminator value
	Type actualType = _discriminatorConvention.ReadDiscriminator(ref reader);
	context.converter = (IObjectConverter<T>)_registry.ConverterRegistry.Lookup(actualType);
	}
	else
	{
	context.converter = this;
	}

But IObjectConverter<T> is restricted to class

Dahomey.Cbor/src/Dahomey.Cbor/Serialization/Converters/ObjectConverter.cs

Lines 22 to 23 in 527b72f

	public interface IObjectConverter<out T> : IObjectConverter
	where T : class