Moved KMS code examples into KMS directory; added new KMS code examples (#480)

* Moved KMS code examples into KMS directory; added new KMS code examples

* Removed stale KMS Cargo.toml file

* Renamed KMS helloword.rs as kms-helloworld.rs; added doc comments to it

* Refactored client creation to omit custom connection

Co-authored-by: Russell Cohen <rcoh@amazon.com>