Ergonomics improvements & a working example (#204)

* Ergonomics improvements & a working example

This diff adds several `pub use` statements to generated service crates to improve ergonomics, namely, this removes the need for customers to depend on internal crates like `aws-auth` when using the SDK. A few other minor bugs were also fixed on the path to getting a working example.

* Fix broken intra doc link

* Fix stale import

* Remove dead code