Regional support for AWS::BillingConductor::* resources

### Name of the resource

Other

### Resource name

AWS::BillingConductor::*

### Description

CloudFormation supports AWS Billing Conductor by supplying the `AWS::BillingConductor::*` resources. Billing Conductor is a global service (similar to IAM, Organizations, etc. as far I personally can tell). However, it is only possible to deploy CloudFormation templates containing billing conductor resources when using the region `us-east-1`.

It would be great to have regional support for the global AWS Billing Conductor resources in order to avoid having to pass around data between CloudFormation stacks.

My use-case:
I am deploying my AWS Organizations tree alongside other components of my basic Organizations setup to my payer account in `eu-central-1` using AWS CDK. I would like to deploy my Billing Conductor setup in the same stack and be able to dynamically reference the accounts the conductor is handling through the CfnAccount's ' `.ref` field so that I don't have to use account IDs in my config anywhere.

### Other Details

The current errors in `eu-central-1` are as follows:
* Invalid or unsupported Type AWS::BillingConductor::PricingRule for resource xyz in eu-central-1
* Invalid or unsupported Type AWS::BillingConductor::PricingPlan for resource xyz in eu-central-1
* Invalid or unsupported Type AWS::BillingConductor::BillingGroup for resource xyz in eu-central-1

Start by reviewing the AWS::BillingConductor::* resource types and the reported behavior in us-east-1 versus eu-central-1. Check how the AWS CDK CfnAccount ref is intended to work across stacks. Done means the listed Billing Conductor resources are accepted in eu-central-1 and the account references can be used without hard-coded account IDs.

Written by the indexing model from the issue text.