[Codegen] KnownValues for ExtensibleEnum treated as string

[HarshaNalluru]

modelId is a string here, can assume existing prebuilt models or new custom models.

With no way of even knowing the known prebuilt values, it is hard to know what to put there unless you copy from a sample.

User example being

  const initialResponse = await client
    .path("/documentModels/{modelId}:analyze", "prebuilt-invoice")
    .post({
      contentType: "application/json",
      body: {
        // The Document Intelligence service will access the following URL to an invoice image and extract data from it
        urlSource: "https://raw.githubusercontent.com/Azure/azure-sdk-for-js/main/sdk/formrecognizer/ai-form-recognizer/assets/invoice/sample_invoice.jpg",
      }
    });

[qiaozha]

documentation missing.