Add nullable struct support to BindAsync

[halter73]

Today, if you want to support an optional parameter with BindAsync, you cannot use a nullable struct. Instead you must use a reference type.

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

// cloudEvent will be inferred to come from a Json request body because no BindAsync method will
// be found on Nullable<CloudEventStruct> and RequestDelegateFactory doesn't try to unwrap the nullable.

app.MapGet("/webhook", (CloudEventStruct? cloudEvent) =>
{
    redis.Publish(cloudEvent);
});

app.Run();

public struct CloudEventStruct
{
	// This will NOT be matched because it returns returns a ValueTask<CloudEventStruct?>
    // instead of ValueTask<CloudEventStruct>
    public static async ValueTask<CloudEventStruct?> BindAsync(HttpContext context, ParameterInfo parameter)
    {
        return await CloudEventParser.ReadEventAsync(context, parameter.Name);
    }
}

This tracks adding support for nullable-struct parameters and nullable-struct-returning BindAsync methods.