Allow FormData to be passed as input to AI bindings

[thatsKevinJain]

Models like bfl/flux-2-dev accept FormData as inputs, where the user
can pass multiple images as style references. This PR allows AI bindings
to pass FormData as-is to the model.

[JoaquinGimenez1]

Why not use something like this?

function isFormData(obj: unknown): obj is FormData {
  return obj instanceof FormData;
}

[thatsKevinJain]

"instanceof" depends on how the object was constructed and can fail to work sometimes.

Referring to the SO article here:
https://stackoverflow.com/a/47204039

The approach used in PR above looks verbose, but it is trying to check if some specific functions of FormData object are present as well.

This is similar to how we check for readable streams:
https://github.com/cloudflare/workerd/pull/5590/files#diff-2fe1e717c62f1145cade283ccc2dabf84edd49d4517103ddc75171c17e5e710fR90

[JoaquinGimenez1]

If we are saying that we accept FormData then the users should build the object as a FormData and not a random object with append and entries methods. This means that the following object will pass the validation

const fakeFormData = {
  append(key: string, value: string) {
    console.log(`Appending ${key}: ${value}`);
  },
  entries() {
    return [['key1', 'value1']];
  }
};

On a side note: I also don't agree how we check for readable streams either

[thatsKevinJain]

@danlapid Do you have ideas on what can be right approach here?