Consider a way to get the operation name from the poller

When we create a poller, such as in:

https://github.com/googleapis/google-cloud-rust/blob/9f00dffe46fe5d50d9f7e3181c7e65d85be4ac8d/guide/samples/src/lro.rs#L118-L139

One may want to cancel the polling operation after a few iterations, or otherwise save it for future use. We need a way to get the operation name, and maybe even a way to start a poller given the operation name (and the types).

For this issue we need to determine if such a change would require a breaking change or not. If it does not require a breaking change, we save the design and we can remove this issue from the milestone. If we do need a breaking change then we need to fix it.