Move pageable helper back to autorest.csharp

[m-nash]

After this PR #3674 it created a situation where we cannot update the signatures for Pageable helper in back compat way because autorest.csharp opens the signatures but now cannot change them since they live in azure.core.

Temporarily moving this back to unblock serialization work here #3434.

Relies on this merging into main first Azure/azure-sdk-for-net#38399

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[jsquire]

Approving based on off-line cooperation. This is needed to unblock other efforts, but will need to be addressed in the longer-term to keep shared source unified in Azure.Core.