ARM Template locks

I'd like the ability to lock a keyvalue at creation and/or after creation.

Perhaps something like this (see `.properties.locked`):

```json
{
"type": "Microsoft.AppConfiguration/configurationStores/keyValues",
"apiVersion": "2020-07-01-preview",
"name": "[concat(variables('configStoreName'), '/environment:name')]",
"dependsOn": [
"[variables('configStoreName')]"
],
"properties": {
"value": "[parameters('environmentName')]",
"contentType": "text/plain",
"tags": "",
"locked": true
}
},
```

The alternative is that I'm able to apply resource locks to configuration keyvalues, but this would also be useful in addition to the above method.

```json
{
"type": "Microsoft.Authorization/locks",
"apiVersion": "2016-09-01",
"name": "environmentNameLock",
"scope": "[resourceId('Microsoft.AppConfiguration/configurationStores/keyValues', variables('configStoreName'), 'environment:name')]",
"dependsOn": [
"[resourceId('Microsoft.AppConfiguration/configurationStores/keyValues', variables('configStoreName'), 'environment:name')]"
],
"properties": {
"level": "ReadOnly",
"notes": "Config must not be modified or deleted."
}
},
```