Add remaining convenience (strongly typed) client surfaces: Fine Tuning, Batch

[trrwilson]

As of beta.3, FineTuningClient and BatchClient only support protocol method use. This mechanism is available for all operations and is a fully valid mechanism to interact with the service, but the binary-in, binary-out pattern isn't nearly as idiomatic to use as convenience method support with strong types.

This issue tracks those remaining scenario clients:

• Convenience layer for BatchClient
• Convenience layer for FineTuningClient