Refactor `services.AccessMFAParams` and the accompanying `MFAParams` methods so
they may be extended to carry additional data (eg, information required to
perform device trust checks).
This PR refactors existing code without adding any new functionality, so it may
be backported to older branches to alleviate eventual conflicts. Follow up
changes will take advantage of it to add support for device trust.
Related to gravitational/teleport.e#514.
* 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