Add a cloudformation export so the WebACL can be referenced in other stacks

[matthewhembree]

Is your feature request related to a problem? Please describe. I'd like to be able to add the WebACL association through automation.

Describe the feature you'd like Create a cloudformation export of the WebACL ARN that can be used in other stacks (and Terraform through data.aws_cloudformation_export.my-webacl).

e.g.:

Type: AWS::WAFv2::WebACLAssociation
Properties: 
  ResourceArn: !Ref MyApiGateway.Arn
  WebACLArn: Fn::ImportValue:
    !Sub "${WAFStackName}-WebAclArn"

Additional context Adding the example resource to my SAM deployments would ensure guaranteed attachment to all deployments (even feature/preview deployments).

[matthewhembree]

Fixed in #121

[aijunpeng]

This issue has been addressed in version >=4.0.0.