[Files] Add pagination convenience methods for OpenAIFileClient.GetFiles

[ShivangiReja]

The latest spec updates GetFiles from non-paginating to paginating by adding three optional parameters: limit (long?), order (string), after (string)

• Add support for pagination convenience methods for OpenAIFileClient.GetFiles. Convenience methods for GetFiles are GA, so we should introduce new overloads(non-breaking) that expose pagination methods for GetFiles while keeping the current signatures intact.

• With the v2.9.0 release, paginated protocol-level overloads were added for OpenAIFileClient.GetFiles . Update these protocol methods to return the pagination types(CollectionResult/AsyncCollectionResult)