Move `Pager` to `azure_core` since it's Azure-specific

[heaths]

As came up during a REST API LT meeting, paging is Azure-specific and the Pager<T> should be moved back into azure_core. Will have to see how that impacts the overall response model i.e., does it try to create any pager when deserializing? I'm pretty sure not, but use it as an example of things to check. /cc @jhendrixMSFT @JeffreyRichter

[JeffreyRichter]

Yes, paging & LROs is Azure-specific.

[heaths]

I knew LROs were. Those are still in azure_core. I'll just need to move paging...if I can (pretty sure I can).