allow build flag --use_external_dawn

[fs-eire]

Description

introduce this build flag --use_external_dawn to allow specifying Dawn as an external dependency. When it is specified, dawn::native is excluded from build of onnxruntime, and passing in dawn proc table is required.

I didn't find a good way to run the test. Currently I need to build with the following flags:

C:\code\onnxruntime>build --use_webgpu --use_external_dawn --skip_tests --target onnxruntime_webgpu_external_dawn_test

And run the test using:

# run test that passing dawn proc table. expect session to initialize successfully
C:\code\onnxruntime\build\Windows\Debug\Debug>onnxruntime_webgpu_external_dawn_test.exe

# run test that does not pass proc table. expect exception to throw.
C:\code\onnxruntime\build\Windows\Debug\Debug>onnxruntime_webgpu_external_dawn_test.exe --no_proc_table