AWS::EC2::Instance - DomainJoinDirectory

## 1. Title

AWS::EC2::Instance-DomainJoinDirectory

## 2. Scope of request

Add the DomainJoinDirectory option to EC2 instances.

## 3. Expected behavior

When launching a new Windows EC2 instance, there should be an option to select an existing AWS Directory Service directory to join the instance to. This functionality already exists in the CLI and the GUI.

## 4. Suggest specific test cases

When automating Windows server launch, it's helpful to have the server already joined to the existing domain before doing further configuration.

## 5. Helpful Links to speed up research and evaluation

AWS Docs:
https://docs.aws.amazon.com/directoryservice/latest/admin-guide/launching_instance.html?icmpid=docs_ec2_console
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-instance.html

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

Compute

## 7. Any additional context (optional)