webgpu: add MatMulBnb4 contrib op support (#29587)
Implement the MatMulBnb4 quantized matmul operator for the WebGPU
execution provider, supporting both FP4 (quant_type=0) and NF4
(quant_type=1) blockwise 4-bit weight dequantization.
### Description
<!-- Describe your changes. -->
Add WebGPU execution provider support for the MatMulBnb4 contrib
operator, enabling blockwise 4-bit quantized matmul (bitsandbytes-style
FP4 and NF4) to run on WebGPU.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Models quantized with bitsandbytes 4-bit weights fell back to CPU when
running on WebGPU, causing significant slowdowns, and cannot use graph
capture feature.
Adding a native WebGPU kernel keeps these quantized MatMulBnb4 on the
GPU, avoiding costly EP fallback and CPU/GPU data transfers.
marqo-fashionSigLIP-text-bnb4: ~30% faster
marqo-fashionSigLIP-vision-bnb4: ~20% faster
