* Add a new db engine
* Add tests for new engine
* Update tsh db subcommands
* Refactor error message and suggestions for unsupported tsh commands
* Add dynamodb to test plan
* Add AWS external ID to db config and update protos
When the AWS Account ID is provided, we must check that it is a valid
one.
AWS Account ID is a well documented field: 12-digit string
This check is meant to prevent typos when creating Databases mainly from
the Web UI in the context of Teleport Discover.
Gavin Frazar