[Feature Request] [webgl] hope webgl AveragePool support ceil mode

### Describe the feature request

Thanks for your great work on web inference.
AveragePool's ceil mode is a necessary feature for our model to run on webgl backend, I saw this a TODO feature for a long time, so is there any chance to make up it recnetly

### Describe scenario use case

Unet like model, require feature concat from downsampled up layer and upsampled down layer

The request concerns AveragePool ceil-mode support in the WebGL backend for an Unet-like model, but it names no implementation file or test. Start by locating the WebGL AveragePool code and its existing TODO, then compare its behavior with the model scenario; done means the model can run with ceil mode enabled.

Written by the indexing model from the issue text.