Update long-running operation APIs to use LRO subclient pattern

[annelo-msft]

Move polling operation endpoints to the SCM-based client LRO pattern. Specifically:

• Protocol and convenience methods for long-running operations return public subtypes of SCM OperationResult (operation subclients)
• Where convenience layer is available, operation subclients add strongly-typed Value and/or Status properties, as applicable to the service operation
• Operation subclients have protocol and convenience methods for any "linked operations" -- i.e. those used to update, resume, or cancel the operation, or retrieve values related to the specific operation
• Operation subclients add overloads to WaitForCompletion and WaitForCompletionAsync that take a polling interval
• Operation subclients have static Rehydrate and RehydrateAsync methods that can be called to recreate an operation subclient from a provided rehydration token

[annelo-msft]

@trrwilson, seek your guidance on getting the AOAI CI to pass ...