Allow Dynamic References for Secure Values in Custom Resources

# Scope of request

Currently, you can use Dynamic References for Secure Values, i.e. `{{resolve:ssm-secure:IAMUserPassword:10}}` in most places. However, it is not possible to use them as parameters (read: arguments) to a Custom Resource.

This is documented here: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/dynamic-references.html#dynamic-references-ssm-secure-strings

Under Addition Considerations:

* Dynamic references for secure values, such as ssm-secure and secretsmanager, are not currently supported in custom resources.

## Samples:

```
AWSTemplateFormatVersion: "2010-09-09"
Resources:
MyFrontEndTest:
Type: "Custom::PingTester"
Version: "1.0"
Properties:
ServiceToken: "arn:aws:sns:us-east-1:84969EXAMPLE:CRTest"
Password: {{resolve:ssm-secure:IAMUserPassword:10}}
Outputs:
CustomResourceAttribute1:
Value:
Fn::GetAtt:
- MyFrontEndTest
- responseKey1
CustomResourceAttribute2:
Value:
Fn::GetAtt:
- MyFrontEndTest
- responseKey2
```

## Expected Behavior:

We should be allowed to use a dynamic reference for secure values on custom resources.

## Helpful links:

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/dynamic-references.html#dynamic-references-ssm-secure-strings
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-cfn-customresource.html

## Category:
Compute, Security