Support CloudFormation intrinsic functions

[Chriscbr]

Currently, trying to use an intrinsic function in most places where a specific value is expected (such as a string) results in an error:

AppSyncEventBridgeRole:
  Type: aws-cdk-lib.aws_iam.Role
  Properties:
    assumedBy:
      aws-cdk-lib.aws_iam.ServicePrincipal:
        service:
          Fn::Sub:
            - 'appsync.${Domain}.com'
            - Domain:
                Ref: amazonaws

which produces the following error:

TypeError: (displayHint || "TOKEN").replace is not a function

Instead, it should generate the equivalent function in the core API (https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.Fn.html).

This should support using both the "long form" (e.g. Fn::Sub) and "short form" (e.g. !Sub -- only available in YAML) of each intrinsic function. These are documented on the pages about intrinsic functions: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-join.html

---

Note: in some rare cases it does work, but I think it's just because CDK doesn't check the values passed to environment:

connectFunction:
  Type: aws-cdk-lib.aws_lambda.Function
  Properties:
    handler: index.handler
    runtime: NODEJS_14_X
    timeout:
      seconds:
        amount: 300
    code:
      fromInline:
        code: exports.handler = function() { console.log("hello world"); }
    memorySize: 256
    environment:
      ENV_VAR:
        Fn::Join:
          - "arn:aws:lambda"
          - ":"
          - "us-east-1"
          - ":"
          - "111122223333"
          - ":function:"
          - "my-function"

For the implementation, consider checking out these related PRs in the AWS CDK:

• fix(diff): handle YAML short-forms like '!GetAtt' in diff aws/aws-cdk#10381
• feat(cfn-include): add support for YAML templates aws/aws-cdk#8746
• chore: deprecate @aws-cdk/yaml-cfn aws/aws-cdk#14001

---

Split out to #270

Also, trying to reference one of the "short forms" in a template immediately causes an error during parsing:

(node:71590) YAMLWarning: The tag !Sub is unavailable, falling back to tag:yaml.org,2002:seq

[mrgrain]

Closing in favor of #275