[enhancement]: Checkbox to feather raster layers

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Contact Details

magnusviri on discord

### What should this feature add?

I would like more control over blending in-painted raster views. Right now, I paint an inpaint mask over the area I want blended it. On the backend Invoke generates a whole frame but then removes everything that isn't part of the mask and blends the edges. I would like more control because I often realize my inpaint mask could've been done better.

Ideally, each raster layer could have a paintable mask like Photoshop. But there is a much easier way. What if you add a checkbox to each raster layer in the canvas view. The checkbox enables a dynamic feathered edges for the raster view. That way I can ignore inpaint masks entirely and I can generate full 1024x1024 layers, click the checkbox, then erase the parts of the layer I don't like. It will automatically feather in with the layer beneath it.

I know this is possible with a soft edged erase, but you don't have soft edge eraser either. I just think the checkbox is an easy way to give users much more control over what is feathered in.

This is one way of implementing a manual version of this [feature request](https://github.com/invoke-ai/InvokeAI/issues/66).

### Alternatives

_No response_

### Additional Content

_No response_