"az ml computetarget attach aks" should validate Kubernetes version

### Extension name (the extension in question)
Machine Learning CLI extension version 1.0.10

### Description of issue (in as much detail as possible)

-----

The `az ml computetarget attach aks` will report success even if the Kubernetes version of the provided AKS cluster is outside the supported range. The problem can only be discovered later using e.g. `az ml computetarget show`:

```
> az ml computetarget attach aks -n test-aksct -i /subscriptions/e60ce6ff-b000-482a-9db3-1eeb766ac2ff/resourcegroups/test-rg/providers/Microsoft.ContainerService/managedClusters/test-aks -w test-mlsw -g test-rg --verbose
Attaching compute resource...
Resource attach submitted successfully.
To see if your compute target is ready to use, run:
az ml computetarget show -n test-aksct

> az ml computetarget show -n test-aksct -w test-mlsw -g test-rg --verbose
{
"location": "eastus",
"name": "test-aksct",
"provisioningErrors": [
{
"code": "BadRequest",
"error": {
"code": "BadRequest",
"details": [
{
"code": "BadRequestFormat",
"message": "Specified kubernetes version 1.12.4 is not supported. Supported versions are between 1.8 - 1.11 "
}
],
"message": "The request is invalid",
"statusCode": 400
},
"message": "The request is invalid"
}
],
"provisioningState": "Failed"
}

```

The compute target gets attached, but is in an unusable state and needs to be detached manually.

The user experience would be much better if `az ml computetarget attach aks` validated the Kubernetes version immediately and returned an error, instead of silently creating an unusable compute target.

Start by reproducing the `az ml computetarget attach aks` command with an AKS cluster outside the supported Kubernetes range, then compare it with `az ml computetarget show`. Trace the attach AKS CLI entry point and make the command reject unsupported versions immediately, without creating an unusable compute target.

Written by the indexing model from the issue text.