[Proposal] Add ThumbnailImageSource and OnBuffering Properties

### Feature name

Add ThumbnailImageSource and OnBuffering Properties

### Link to discussion

https://github.com/CommunityToolkit/Maui/issues/1749

### Progress tracker

- [X] Android Implementation
- [X] iOS Implementation
- [X] MacCatalyst Implementation
- [X] Windows Implementation
- [ ] Tizen Implementation
- [ ] Unit Tests
- [x] Samples
- [ ] Documentation

### Summary

This proposal if for adding two properties:

1. "ThumbnailImageSource" This property can be use to show thumbnail image when video is not loaded.
2. "OnBuffering" This Property will be an event which will trigger when the video is buffering. Using this we can show/hide activity indicator when this event triggers.

### Motivation

This can improve the user experience and enhance the video rendering

### Detailed Design

public class MediaElement
{

public static readonly BindableProperty ThumbnailImageSourceProperty;
public static readonly BindableProperty OnBufferingProperty;

public ImageSource ThumbnailImage
{
get ;
set ;
}

public EventHandler OnBuffering
{
get ;
set ;
}
}

### Usage Syntax

```markdown

```

### Drawbacks

_No response_

### Alternatives

_No response_

### Unresolved Questions

_No response_
```[tasklist]
### Tasks
```

Start with the MediaElement API described in the proposal and review the existing Android, iOS, MacCatalyst, and Windows implementations, then compare the remaining Tizen work. Done means the missing platform support, unit tests, and documentation are completed while preserving the listed samples and intended thumbnail and buffering behavior.

Written by the indexing model from the issue text.