bug: resource translation fails when a `ResourceValue::Object` containing a nested `TypeReference::Union` is encountered in deserialized template

[colifran]

When a deserialized input template has a property that is a ResourceValue::Object containing a nested TypeReference::Union, the resource translation will fail because it is currently unimplemented - https://github.com/cdklabs/cdk-from-cfn/blob/fd3ec9e73278d9cc9b476d43140cfb077dad3963/src/ir/resources/mod.rs#L133

As an example, consider the following template:

ClientFunction:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: 'pricing-client'
      Handler: client_lambda.lambda_handler
      MemorySize: 512
      Timeout: 300
      CodeUri: ./pricing-client/
      PermissionsBoundary: !Sub 'arn:aws:iam::${AWS::AccountId}:policy/GithubActionsIamResourcePermissionsBoundary'
      Role: !GetAtt LambdaExecutionRole.Arn
      Runtime: python3.11
      Events:
        ApiEvent:
          Type: Api
          Properties:
            Path: /path
            Method: get

The Events property is an event source that must contain one of the following event types: AlexaSkill, Api, CloudWatchEvent, CloudWatchLogs, Cognito, DocumentDB, DynamoDB, EventBridgeRule, HttpApi, IoTRule, Kinesis, MQ, MSK, S3, Schedule, ScheduleV2, SelfManagedKafka, SNS, SQS - https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-property-function-eventsource.html. As a result, the corresponding type is a TypeReference::Union(TypeUnion::Static([TypeReference::Named(_), ..])) which translates to meaning that the event type value must be one of the event types listed above.

We should add support for the TypeReference::Union type. This could be supported via a UnionOf struct similar to the MapOf struct used for the TypeReference::Map type. Further, we need to consider how we could collapse the union type down to a specific TypeReference::Named type based on the specific value detected.