Lambda function URLs support

[Function URLs](https://aws.amazon.com/blogs/aws/announcing-aws-lambda-function-urls-built-in-https-endpoints-for-single-function-microservices/) were announced today. It'd be great to support them in aws-lambda-go, just like [API Gateway](https://github.com/aws/aws-lambda-go/blob/main/events/README_ApiGatewayEvent.md).
