Proposal: Add support for BorderMode in BlurEffect

## Summary
The proposal is to add support for **`BorderMode`** in **BlurEffect**

This will enable blur effect to be more visually comforting and will enable UI designers to be even more creative.

## Goal
The goal is to add an enum for **`BorderModes`** and a property called **`BorderMode`** in **BlurEffect** class (and internal work that I'm not aware of).

## Rationale
Currently, **BlurEffect** pads extra pixels. Which is good for **UIElements** that are neither clipped nor stretched to fit parent.
But will look ugly when applied to **UIElements** which are either clipped or stretched to fit parent container
See this SO question : [Gaussian blur leads to white frame around image](https://stackoverflow.com/questions/6236054/gaussian-blur-leads-to-white-frame-around-image) to get an insight of my proposal.

**In the image below the element is blurred and has extra padded pixels. It's ok in this case**


**But for cases like this where the element is either stretched to fill or clipped, it will have ugly transparent edges**


So having an option to not pad extra pixel will be helpful in many cases

UWP (Win2D?) has the said property see [GaussianBlurEffect.BorderMode](https://microsoft.github.io/Win2D/html/P_Microsoft_Graphics_Canvas_Effects_GaussianBlurEffect_BorderMode.htm)
and also [Direct2D/GaussianBlur.BorderMode](https://docs.microsoft.com/en-us/windows/win32/direct2d/gaussian-blur#border-modes)

## Proposed API

The same API found in Win2D's GaussianBlurEffect can be used.

```cs
// Enumeration type that specifies how to process pixels that lie outside the input image.
public enum EffectBorderMode
{
// Pixels outside the input image are treated as transparent black, resulting in a soft edge after processing.
Soft = 0, // Default
// The effect clamps to the size of the input image.
Hard = 1
}
```

and in BlurEffect.cs

```cs
// Gets and sets border mode for edge pixels.
public EffectBorderMode BorderMode { get; set; }
```

## Extra Info
_I'm a new comer to WPF and not well introduced to **Open-Source workflow**.
If I there are any mistakes in this proposal, please correct me._

One thing that bothers me is that it's been 13 years since WPF's release and I can't find any issue proposing to change this behavior. **Is it because this can't be implemented or no one asked for it?**

At this point I'm too afraid to ask this, since WPF is on maintenance mode and we are forced towards WinUI for a stable future.
I love WPF and people like me will be so pleased if WPF can be improved with much more features.

## Open questions
- Is this even feasible?
- What will be the difficulty of implementation?
- Is this out of scope?
- Do you devs still plan on adding new features?
- If it the implementation process is feasible, will you consider implementing it?
- Doesn't WPF and UWP both use DirectX for rendering? Then is it possible?