WebNN: Add a switch to configure the ORT graph optimization level

According to the ORT doc[1], ONNX Runtime provides various graph
optimizations which are essentially graph-level transformations. This
CL adds a WebNN switch to configure the graph optimization level which
supports testing different levels at runtime and analyzing the
performance.

The usage is: `chrome.exe --enable-features=WebNNOnnxRuntime
--webnn-ort-graph-optimization-level=DISABLE_ALL`

If the switch is not passed or passed with an invalid value, the
default level `ORT_ENABLE_ALL` will be applied.

[1] https://onnxruntime.ai/docs/performance/model-optimizations/graph-optimizations.html

Bug: 416543902,412841630
Change-Id: I98bc709f0b5a54631f64aa288efbc4d60ec4ca52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6823312
Reviewed-by: Bo Liu <boliu@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Commit-Queue: Mingming1 Xu <mingming1.xu@intel.com>
Reviewed-by: ningxin hu <ningxin.hu@intel.com>
Cr-Commit-Position: refs/heads/main@{#1500536}