AWS::SecurityHub::CustomActionTarget - Manage resources

## 1. Title
#### AWS::SecurityHub::CustomActionTarget

## 2. Scope of request
This issue is to add the ability to manage SecurityHub Custom Action Targets. This would allow the action target to be managed in the same manner as the CloudWatch Events that the target would trigger.

## 3. Expected behavior
Normal CFN resource behaviour.

## 4. Suggest specific test cases
Normal CFN resource test cases.

## 5. Helpful Links to speed up research and evaluation
* AWS CLI: https://docs.aws.amazon.com/cli/latest/reference/securityhub/index.html
* Boto3 API: https://boto3.amazonaws.com/v1/documentation/api/1.10.45/reference/services/securityhub.html

## 6. Category (required) - Will help with tagging and be easier to find by other users to +1
Security (IAM, KMS...)

## 7. Any additional context (optional)

Start with the AWS CLI SecurityHub documentation and the linked Boto3 API reference to determine the Custom Action Target resource properties and operations. Review the project's normal CloudFormation resource test cases, then define completion as managing the target with standard resource behavior and corresponding tests.

Written by the indexing model from the issue text.