Support Page<T> in LRO response type in CADL

[lirenhe]

This issue was reported in dotnet https://github.com/Azure/autorest.csharp/issues/2905, but I expect Java DPG should also support this. Below is an example CADL with LRO POST action that generates a pageable collection of file URIs.

  #suppress "@azure-tools/cadl-azure-core/use-standard-operations" "ADP data-plane API uses custom LRO response template the LRO actions."
  @doc("Initiates the process of generating SAS signed URIs for accessing the data-stream files.")
  @extension("x-ms-long-running-operation", true)
  @asyncOperationOptions("location")
  @pollingOperation(LongRunningOperationStatus)
  generate is CustomLongRunningResourceCollectionAction<
    DataStreamFile,
    {},
    Page<DataStreamFile>
  >;

[weidongxu-microsoft]

mgmt generated code https://github.com/Azure/azure-sdk-for-java/blob/main/sdk/resourcemanager/azure-resourcemanager-network/src/main/java/com/azure/resourcemanager/network/implementation/NetworkManagementClientImpl.java#L2279

However, mgmt leaves out the LRO part of the API, it only generate the pageable API

PagedFlux<BastionShareableLinkInner> putBastionShareableLinkAsync

not the LRO + pageable

PollerFlux<T, PagedFlux<BastionShareableLinkInner>> beginPutBastionShareableLinkAsync

(I am not sure whether this thing is even valid)

So, basically we do not know what be the method signature.

[weidongxu-microsoft]

ADP removed this. So lower priority.

Discussed with Srikanta, actually we'd like API arch and SDK arch to have guideline on this case. Currently there is no guideline on LRO + pageable, and it is rarely used (if ever) in data-plane.