Storage Cache: tags should be a map<string, string>

Storage Cache has the following defition of [tags](https://github.com/Azure/azure-rest-api-specs/blob/27cc07ddd294d98e05cb301e07a72378df9f87e8/specification/storagecache/resource-manager/Microsoft.StorageCache/stable/2020-03-01/storagecache.json#L1201):

```
"tags": {
"description": "ARM tags as name/value pairs.",
"type": "object"
},
```

Instead, it should mark the type of the values as `string`:

```
"tags": {
"description": "ARM tags as name/value pairs.",
"type": "object",
"additionalProperties": {
"type": "string"
}
},
```

For example, see this definition of the [storage accounts](https://github.com/Azure/azure-rest-api-specs/blob/27cc07ddd294d98e05cb301e07a72378df9f87e8/specification/storage/resource-manager/Microsoft.Storage/stable/2019-06-01/storage.json#L2210-L2214).