Add support for an execute operation that passes the client library as a callback parameter

[markpollack]

When an operation against the deployer API is not sufficient to implement a use-case, having access to the underlying platform client library for a SPI provider simplifies the app's implementation, by not having to create another separate platform client instance.

The model in RabbitOperations can serve as a template, e.g.

<T> T execute(ClientCallback<T> action) 

Each SPI implementer can parameterize this method with the type of the platform client.