onnxruntime

[js] enable external data loading for ort-web (#19087)

### Description
enable external data loading for ort-web.

### Why
The ORT external data design is highly depending on the file system,
especially synchronous file I/O APIs. Those are not available in web
platforms. We need to have extra code to make external data working on
web.

### How
Considering there is no file system in web, an implementation for web to
support external data is to use pre-loaded data. Assume model file
a.onnx includes initializers that linked to ./b.bin, we require users to
pass a full data file list when creating the session. The user code will
be look like:
```js
const mySess = await ort.InferenceSession.create('./path/model/a.onnx', {
  // session options
  externalData: [
    {
      // relative or absolute path/URL of the file,
      // or a pre-loaded Uint8Array containing the data of the external data file
      data: './path/data/b.bin', 

      // the relative path of the external data. Should match initializers' "location" value defined in the model file
      path: './b.bin'
    },
    // { } if multiple external data file
  ]
});
```

Currently, this feature only works with JSEP build enabled.