[new rule]: In context of model, x-ms-flatten is allowed only in property name

### Lint rule description

x-ms-flatten has a clear scope in models, and should be used only inside a property definition

### Related swagger example

This is incorrect:
https://github.com/Azure/azure-rest-api-specs/blob/70c71be6e655ecca5dff4ef7f53438e6755e3e70/specification/eventhub/resource-manager/Microsoft.EventHub/stable/2021-11-01/namespaces-preview.json#L737

### Category

SDK

### Severity level

Error

### Applies to

Management plane API spec, Data plane API spec, RPaaS API spec

### How to fix the violation

Move the node to right place of what you want to flatten

### What't the impact if breaking the rule

Autorest is silently ignoring the property, which leads to confusion and people thinkings autorest is buggy