Add two Amazon Polly examples (#286)

* Add to Amazon Polly examples

* Fix copy-paste error

* Update aws/sdk/examples/polly-helloworld/src/main.rs

Co-authored-by: David Barsky <dbarsky@amazon.com>

* Update aws/sdk/examples/polly-helloworld/src/main.rs

Co-authored-by: David Barsky <dbarsky@amazon.com>

* Some cleanups

* Rerun precommit

* CR Feedback

* Fix clippy lint

* Small cleanup

Co-authored-by: David Barsky <dbarsky@amazon.com>