`Microsoft.AppConfiguration/configurationStores` - `sku.name` should be enum type

https://github.com/Azure/azure-rest-api-specs/blob/06bf50ba45fe22722dfc6340b58fe45a3a71f7c6/specification/appconfiguration/resource-manager/Microsoft.AppConfiguration/stable/2023-03-01/appconfiguration.json#L2069-L2081

By testing the API the `sku.name` property only supports `free` and `standard`. We should use Enum for the property instead of json.

Open specification/appconfiguration/resource-manager/Microsoft.AppConfiguration/stable/2023-03-01/appconfiguration.json at lines 2069-2081 and inspect the sku.name property. Confirm the API-supported values are free and standard, then update the schema to represent those values as an enum and run the repository's specification validation. Done means the property no longer accepts arbitrary JSON values.

Written by the indexing model from the issue text.