This PR adds the ability to automatically create references in
cross-region stacks. You can now do something like

```ts
const stack1 = new Stack(app, 'Stack1', { env: { region: 'us-east-1' } });
const cert = new certificatemanager.Certificate(stack1, 'Cert', {...});

const stack2 = new Stack(app, 'Stack2', { env: { region: 'us-east-2' } });
new cloudfront.Distribution(stack2, 'Distro', {
  certificate: cert,
})
```

To accomplish this, I've added a new construct `ExportsReader` which
creates a Lambda backed custom resource. This custom resource will
return all of the CloudFormation exports in a given region.

Given the above example, this would create an output in `stack1`

```json
{
  "Outputs": {
    "ExportsOutputRefCert5C9FAEC110AE5C6A": {
      "Value": {
        "Ref": "Cert5C9FAEC1"
      },
      "Export": {
        "Name": "East1:ExportsOutputRefCert5C9FAEC110AE5C6A"
      }
    }
  }
}
```

And then an "import" in `stack2` which references the `ExportReader`

```json
{
 "Resources": {
  "Distro87EBE6BA": {
   "Type": "AWS::CloudFront::Distribution",
   "Properties": {
    "DistributionConfig": {
     "ViewerCertificate": {
      "AcmCertificateArn": {
       "Fn::GetAtt": [
        "ExportsReaderuseast1D746CBDB",
        "East1:ExportsOutputRefCert5C9FAEC110AE5C6A"
       ]
      }
    }
   }
  }
 }
}
```

Currently this will create a single ExportsReader per region, but we
could potentially update this to just use a single ExportsReader which
can list exports from a list of regions.

Future extensions:
- Could be updated to do cross account references as well
- Could be used to implement weak references

…ion-provider

- moving feature flag to stack property
- renaming some things