[Web] Missing API for fetching model metadata

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### Describe the issue

In Java, I can do
```java
OrtSession session = env.createSession(res.getContentAsByteArray());
OnnxModelMetadata meta = session.getMetadata();
Map customMeta = meta.getCustomMetadata();
String imgzz = customMeta.get("imgsz");
```

To get the `imgsz` metadata from a model.

In TypeScript, I expected to be able to do the same with roughly the same API, however I cannot find any API for this. Am I missing something, has nobody just gotten around to implementing the API yet (is this what
https://github.com/microsoft/onnxruntime/blob/a5dab850b8dfa64d8818eefa01e0df40cf3e4400/js/common/lib/inference-session.ts#L321-L324 is about?) or is there another reason the API does not exist?

### To reproduce

-

### Urgency

_No response_

### ONNX Runtime Installation

Released Package

### ONNX Runtime Version or Commit ID

1.15.0

### Execution Provider

'wasm'/'cpu' (WebAssembly CPU)

Start with js/common/lib/inference-session.ts around the referenced lines and compare the existing TypeScript web session surface with the Java metadata API shown in the issue. Trace how the WebAssembly CPU session is exposed, then verify that model metadata, including custom metadata such as "imgsz", can be fetched through the web API and add coverage where the existing tests for this surface are located.

Written by the indexing model from the issue text.