Add ToJson for DynamoDBEvent

[ashovlin]

Issue #, if available: #1654, #1657, DOTNET-7345

Description of changes:

Before

In #1648, we removed the dependency on AWSSDK.DynamoDBv2 from Amazon.Lambda.DynamoDBEvents. This allowed us to address nullability issues related to the SDK's marshalling code, which is likely not relevant when working with a Lambda event.

However, if you were working with the SDK's higher level document or object persistence representation of the DynamoDB event, that code may no longer work since the AWSSDK.DynamoDBv2 dependency may no longer be present and we separated the previously shared types.

After

Now, you can convert either the OldImage or NewImage on each DynamoDB event record to json via the new ToJson or ToJsonPretty methods. This can be used with Document.FromJson in AWSSDK.DynamoDBv2 to initialize the higher-level programming models.

foreach (var record in dynamoEvent.Records)
{
    // Convert the event to a JSON string
    var json = record.Dynamodb.NewImage.ToJson();

    // Which you can convert to the mid-level document model
    var document = Document.FromJson(json);
    
    // And then to the high-level object model using an IDynamoDBContext
    var myClass = context.FromDocument<T>(document);
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ashovlin]

CI failure is unrelated to this change, see aws/aws-extensions-for-dotnet-cli#302

[dscpinheiro]

Were you planning to update the package version (https://github.com/aws/aws-lambda-dotnet/blob/master/Libraries/src/Amazon.Lambda.DynamoDBEvents/Amazon.Lambda.DynamoDBEvents.csproj#L9) later?

I also think it'd be good to update the blueprints to use the new version, but I guess we'd need to coordinate that with the toolkit team.

Thanks again for taking a look.

[ashovlin]

Were you planning to update the package version (https://github.com/aws/aws-lambda-dotnet/blob/master/Libraries/src/Amazon.Lambda.DynamoDBEvents/Amazon.Lambda.DynamoDBEvents.csproj#L9) later?

I also think it'd be good to update the blueprints to use the new version, but I guess we'd need to coordinate that with the toolkit team.

Thanks again for taking a look.

Added the version bump for Amazon.Lambda.DynamoDBEvents. I think we'd have to bump the blueprints in a separate PR anyway since those have a package reference that isn't in NuGet yet. I'll add a note to our internal ticket for after this ships.