Moved SSM code examples to ssm directory; add a describe-parameters code example (#501)

* Moved SSM code examples to ssm directory; add a describe-parameters code example

* Renamed ssm-helloworld.rs to create-parameter.rs, as that's what it does

* Refactored response to remove unused parameter

* Updated SSM code examples to use tracing_subscriber::fmt::init()

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