`Purview` Identity Cannot Set to UserAssigned

Identity can only be set to `SystemAssigned` or `SystemAssigned, UserAssigned`, but not `UserAssigned` in the Swagger
https://github.com/Azure/azure-rest-api-specs/blob/32246a0658815042bb73b54b884cb7d87d824beb/specification/purview/resource-manager/Microsoft.Purview/stable/2021-07-01/purview.json#L1170-L1174

API reqeust Detail: (failed with `UserAssigned` but can work with `SystemAssigned, UserAssigned`. On Azure Portal, when choosing `UserAssigned`, the Get response also returns `SystemAssigned, UserAssigned` for identity)
```
PUT request body:
{
// ...
"identity": {
"type": "UserAssigned",
"userAssignedIdentities": {
"/subscriptions/.../resourcegroups/rg/providers/Microsoft.ManagedIdentity/userAssignedIdentities/identity": {}
}
}
}

Error:
{
"error": {
"code": "1001",
"message": "PrincipalId is required",
"target": "PrincipalId",
"details": []
}
}
```

Start with the referenced specification/purview/resource-manager/Microsoft.Purview/stable/2021-07-01/purview.json lines 1170-1174 and compare its identity definition with the provided PUT request and error response. Check the comment thread for the agreed resolution; done means the Purview identity specification accurately reflects the supported UserAssigned behavior.

Written by the indexing model from the issue text.