[Feature Request] Support iGPU/dGPU configuration on WebGPU backend

[wenqinI]

Describe the feature request

In the WebGPU backend, ORT uses HighPerformance as default option:

onnxruntime/onnxruntime/core/providers/webgpu/webgpu_context.cc

Line 46 in dde2fef

req_adapter_options.powerPreference = wgpu::PowerPreference::HighPerformance;

ORT could use some runtime flag or environment to dynamically change the option between HighPerformance and LowPower.

Describe scenario use case

When user want to use iGPU on a machine which already have a dGPU.