[WebNN EP] Add cache for `MLContext`s in the `WebNNBackend` (#22510)
### Description
This change adds a cache of `MLContext`s keyed by their options to the
`WebNNBackend`. This makes is so that multiple `InferenceSession`s
create with the same options will share the same context.
### Motivation and Context
Since `MLTensor`s are tied `MLContext`s, developer can't easily share
tensors between `InferenceSession` (outside of manually an `MLContext`
and specifying the `context` options). This leads strange behaviors such
as,
```js
const sessionsA = ort.InferenceSession.create(urlA, {
executionProviders: ["webnn"],
preferredOutputLocation: "ml-buffer",
});
const sessionsB = ort.InferenceSession.create(urlB, {
executionProviders: ["webnn"],
});
const temp = await sessionA.run({/* arguments */});
const result = await sessionB.run({"input":temp["output"]}); // ERROR: Failed to execute 'dispatch' on 'MLContext': Invalid inputs: The context of MLGraph doesn't match the context of the MLTensor with name "input".
```
We encountered this behavior when updating the transformers.js version
in the developer preview demos. microsoft/webnn-developer-preview#46
onnxruntime
df236c78 - [WebNN EP] Add cache for `MLContext`s in the `WebNNBackend` (#22510)
