Optionally generate partial method or call method optionally when using ObservablePropertyAttribute

### Overview

It's not possible to generate a virtual OnPropertyChanged method right now.
On changing/changed method of property generated as partial.
I want to override changing/changed method by subclass.

### API breakdown

None.

### Usage example

```csharp
[ObservableProperty(GenerateOnChanging = false, GenerateOnChanged = false, CallOnChanging = false, CallOnChanged = false)]
private partial string _fieldA;

[ObservableProperty(GenerateOnChanged = false)]
private partial string _fieldB;

protected virtual void OnFieldBChanged(string? oldValue, string newValue) { }
protected virtual void OnFieldBChanged(string value) { }
```

### Breaking change?

No

### Alternatives

```csharp
[ObservableProperty]
private partial string _field;

partial void OnFieldChanged(string value)
{
OnFieldChanged2(value);
}

protected virtual void OnFieldChanged2(string value) { }
```

### Additional context

_No response_

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