Add support for Paginators (#1006)

* Add Support for Paginators

This commit adds pagination support for both clients. It does this via the `FnStream` abstraction which combines a rendezvous-channel with a closure to produce a stream. When the stream is polled, the underlying closure is polled to advance the computation which then pushes data into the channel.

Two paginators are generated:
1. `.paginate()` which produces a stream of `Result<Page, Err>`
2. `.paginate().items()` which produces a stream of `Result<Item, Err>` where items are flattened from the individual pages. This uses `TryFlatMap` to support conveniently generating a flat page iterator.

In addition, docs are generated to point customers towards the paginators.

* Add RFC, more tests

* backout unrelated changes

* Fix paginators for glacier

* Fix clippy error

* Add test for paginators that point to maps & fix apigateway paginator

* Fix docs

* remove extraeneous println

* Fix DynamoDB examples

* Disallow paginating operations with the idempotency token trait

* Apply suggestions from code review

misc typos / cleanups

Co-authored-by: Zelda Hessler <zhessler@amazon.com>

* CR feedback

* Fix typo

* Update changelog

* Apply suggestions from code review

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* CR feedback round 1

* Rename paginate to into_paginator()

* update pr bot comment

Co-authored-by: Zelda Hessler <zhessler@amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>