Implement custom deserializer for LambdaRequest

[calavera]

This deserializer gives us full control over the error message that we return for invalid payloads. The default message that Serde returns is usually very confusing, and it's been reported many times as something people don't understand.

This code is a copy of the code that Serde generates when it expands the Deserialize macro.

Previous error message:

failed to deserialize the incoming data into the function's payload type: data did not match any variant of untagged enum LambdaRequest

New error message:

failed to deserialize the incoming data into the function's payload type: this function expects a JSON payload from Amazon API Gateway, Amazon Elastic Load Balancer, or AWS Lambda Function URLs, but the data doesn't match any of those services' events

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