[request]: Serialize/Deserialize of models for Lambda events

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**Tell us about your request**

Add support for deserialization of structs for use with Lambda events. This is similar to https://github.com/awslabs/aws-sdk-rust/issues/211, but for event sources that integrate with Lambda.

Alternatively, we could provide a official set of Lambda events, but this would be hard to do without an authoritative repository of event schemas for Lambda.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

I am currently writing a Lambda function in Rust for a [GitHub sample](https://github.com/aws-samples/serverless-rust-demo) that's invoked from a DynamoDB streams to showcase how to do parallel processing within a Lambda function.

Lambda's [Event Source Mapping](https://docs.aws.amazon.com/lambda/latest/dg/invocation-eventsourcemapping.html) takes care of polling events from the DynamoDB stream and invoking the Lambda function with a batch. Lambda sends a JSON objects resembling [`aws_sdk_dynamodbstreams::output::GetRecordsOutput`](https://awslabs.github.io/aws-sdk-rust/aws_sdk_dynamodbstreams/output/struct.GetRecordsOutput.html) without `NextShardIterator`.

**Are you currently working around this issue?**

Right now, I've recreated most of it by making my own `struct`s copying the Go Lambda SDK's [`DynamoDBEvent`](https://github.com/aws/aws-lambda-go/blob/1cb90e0cf5114f9d11568e3aa2322a4f4675447b/events/dynamodb.go#L7-L9) structure.

**Additional context**

N/A

**Attachments**

* [Pull Request (in progress)](https://github.com/aws-samples/serverless-rust-demo/pull/15)

Review the DynamoDB stream event shape in aws-samples/serverless-rust-demo#15 and the Go reference at events/dynamodb.go, then compare it with aws_sdk_dynamodbstreams::output::GetRecordsOutput. Done means defining an agreed way to serialize and deserialize Lambda event structs, with coverage for the requested event shape.

Written by the indexing model from the issue text.