AWS::AutoScaling::AutoScalingGroup - LaunchTemplateSpecification-Version - Add support for specifying $Latest and $Default

## 1. AWS::AutoScaling::AutoScalingGroup-LaunchTemplateSpecification-Version - Add support for specifying $Latest and $Default

The EC2 service allow one to create a Launch Template Specification with a `Version` that is either a version number or one of the keywords: `$Latest` or `$Default` ([docs](https://docs.aws.amazon.com/autoscaling/ec2/APIReference/API_LaunchTemplateSpecification.html)).

However, currently CloudFormation does only support specifying numeric versions for an EC2 Launch Template Specification ([docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-autoscaling-autoscalinggroup-launchtemplatespecification.html#cfn-autoscaling-autoscalinggroup-launchtemplatespecification-version)).

This feature request is about allowing one to specify `$Latest` and `$Default` as well.

```
LaunchTemplateId: String
LaunchTemplateName: String
Version: String # <-- This attribute should also support: "$Latest" and "$Default"
```

## 2. Scope of request
From the [docs:](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-autoscaling-autoscalinggroup-launchtemplatespecification.html#cfn-autoscaling-autoscalinggroup-launchtemplatespecification-version)

> **Version**
The version number. AWS CloudFormation does not support specifying $Latest, or $Default for the template version number.

From trial and error, I can confirm that this is the current behavior.

This request is about resolving this limitation and allow specying `$Latest` and `$Default` for the `Version` attribute.

## 3. Expected behavior

Example use:

```
LaunchTemplateId:
Version: $Latest
```

```
LaunchTemplateId:
Version: $Default
```

## 4. Suggest specific test cases

Test the common use case by passing `$Latest` and `$Default` to the `Version` attribute.
Test drift detection by verifying that it supports the new attribute values.

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

#### EC2 Auto Scaling API Reference (DataType: LaunchTemplateSpecification)
https://docs.aws.amazon.com/autoscaling/ec2/APIReference/API_LaunchTemplateSpecification.html

> **Version**
The version number, $Latest, or $Default. If the value is $Latest, Amazon EC2 Auto Scaling selects the latest version of the launch template when launching instances. If the value is $Default, Amazon EC2 Auto Scaling selects the default version of the launch template when launching instances. The default value is $Default.

#### CloudFormation LaunchTemplateSpecification docs:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-autoscaling-autoscalinggroup-launchtemplatespecification.html#cfn-autoscaling-autoscalinggroup-launchtemplatespecification-version

> **Version**
The version number. AWS CloudFormation does not support specifying $Latest, or $Default for the template version number.

## 6. Category (required)

Compute (EC2)
Compute (EC2 Auto Scaling Groups)
Compute (EC2 Launch Templates)