[BUG] identity id of Eventhub Namespace got changed by the api

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/eventhub/resource-manager/Microsoft.EventHub/preview/2022-10-01-preview/namespaces-preview.json#L884

### API Spec version

2022-10-01-preview

### Describe the bug

when trying to assign the user assigned identity to eventhub namespace, the value passed by the user is: `/subscriptions/85xxx/**resourceGroups**/xiaxintestRG-namespacecmk/providers/Microsoft.ManagedIdentity/userAssignedIdentities/xiaxintestuai-ehn`
however, the api doesn't respect the user's input, changed the camel case to lowercase as below:

### Expected behavior

The format of the id that's returned by the api should be the same as user's input

### Actual behavior

The api changed the id without notifying the user

### Reproduction Steps

create the eventhub namespace and assign the uai

### Environment

_No response_