NBMsgPack037: Async converters should override PreferAsyncSerialization

Custom converters that override the ReadAsync or WriteAsync methods should also override PreferAsyncSerialization and have it return true.

Example violation

The following converter overrides the async methods but doesn't override PreferAsyncSerialization in order to indicate that its async methods are preferred:

internal class MyCustomConverter : MessagePackConverter<SomeCustomType>
{
    public override SomeCustomType? Read(ref MessagePackReader reader, SerializationContext context)
    {
        throw new NotImplementedException();
    }

    public override void Write(ref MessagePackWriter writer, in SomeCustomType? value, SerializationContext context)
    {
        throw new NotImplementedException();
    }

    public override ValueTask<SomeCustomType?> ReadAsync(MessagePackAsyncReader reader, SerializationContext context)
    {
        return base.ReadAsync(reader, context);
    }

    public override ValueTask WriteAsync(MessagePackAsyncWriter writer, SomeCustomType? value, SerializationContext context)
    {
        return base.WriteAsync(writer, value, context);
    }
}

Resolution

Simply add an override that returns true:

public override bool PreferAsyncSerialization => true;