[python] Accept ref to azure.core.exceptions.OdataV4Format in model definition.

transferred from https://github.com/Azure/autorest.python/issues/2853

We know for quite some time that at some point, we would need to support a way to have codegen ref an externally defined type, and not generate code for a model. This could be used to point to an azure-core type, or to a patched type.

Part of this, we discussed with @iscai-msft to support an update in rest_field that would look like this:
```python
details: List[ODataV4Frmat] = rest_field(ref=azure.core.exceptions.OdataV4Format)
```

IOW, a way to explain that this `rest_field` points to something else.

The scope of this issue is to support the first scenario that `Fundations.Error` will ref to `azure.core.exceptions.ODataV4Format`. In the particular case of that class, since `azure.core.exceptions.ODataV4Format` do not support a common deserilization contract, we should have a `if` statement in model base to deserilize properly by calling the constructor.

Long term, we should conclude on the deserialization contract required for ref, and update azure-core ODataV4Format acordingly, but short term a quick hack wouldn't hurt anyone, and it will be transparent for users.

This particular isue, we initiate the ref because we recognized a known type from TSP Azure.Core. Long term, we likely would need a decorator like this:
```
@ref("azure.service.MyType")
property: TypeWeDontGenerate
```
But it's another story for later, and doesn't impact what we're doing here.