[BUG] `NewRelic` - Some types of `identity` are not supported

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/53bed2dccf392c14a412735c38a0f55645812294/specification/newrelic/resource-manager/NewRelic.Observability/stable/2022-07-01/NewRelic.json#L1940

### API Spec version

2022-07-01

### Describe the bug

The `identity` is defined as a [ManagedServiceIdentityType](https://github.com/Azure/azure-rest-api-specs/blob/53bed2dccf392c14a412735c38a0f55645812294/specification/common-types/resource-management/v4/managedidentity.json#L35), which should support `SystemAssigned`,`UserAssigned` and `SystemAssigned, UserAssigned`. However, currently only `SystemAssigned` is supported.

### Expected behavior

`identity` should support `SystemAssigned`,`UserAssigned` and `SystemAssigned, UserAssigned`.

### Actual behavior

Only `SystemAssigned` is supported

### Reproduction Steps

Create a New Relic instance by setting `identity` as `UserAssigned` or `SystemAssigned, UserAssigned`

### Environment

_No response_

Start with the NewRelic.json definition at the linked line and compare its identity property with the referenced managedidentity.json ManagedServiceIdentityType. Confirm the specification represents all three supported identity values, then validate that the New Relic API definition accepts each value.

Written by the indexing model from the issue text.