onnxruntime

Add MaxPool FP16 in XnnPack EP (#22258)

### Description
Add support for FP16 kernels in the XnnPack execution provider for
MaxPool operations.
Fixes:
[AB#50332](https://aiinfra.visualstudio.com/6a833879-cd9b-44a4-a9de-adc2d818f13c/_workitems/edit/50332)

### Motivation and Context
The major purpose of this pull request is to add some common
vars/functions and setup a consistent style for adding FP16 kernels in
XnnPack EP.

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