Moved DynamoDB code examples into DynamoDB directory (#476)

* Moved DynamoDB code examples into DynamoDB directory

* Renamed DynamoDB helloworld code example; added doc comments to all code examples

* Changed CRUD example to use query instead of scan

* Refactored client creation to omit custom connection; cleand up Cargo.toml

* Updated DynamoDB CRUD examples based on feedback

* Removed unused crates from DynamoDB Cargo.toml file

* Updated DynamoDB code examples to use latest patterns.

* Fixed typo in table name in DynamoDB code example

* Updated DynamodB crud code example

* Removed explicit check for existing table in DynamoDB crud code example

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