Complete the sfn versioning API

[andresliszt]

Hey there: Added these methods to stepfunctions

• list_state_machine_versions returns only published versions, ordered newest-first.
• delete_state_machine_version is idempotent when the version does not exist, but raises ConflictException if any alias references the version.
• publish_state_machine_version returns stateMachineVersionArn and creationDate.
• list_state_machine_versions and publish_state_machine_version raise StateMachineDoesNotExist for unknown state machine ARNs.
• create_state_machine rejects versionDescription unless publish=True, raising ValidationException.

Also simplifies a little bit the code creating a helper method _get_state_machine(self, arn: str) -> StateMachine which was repeated a lot across the file.

Simplfies the tests using _get_default_role and simple_definition

[andresliszt]

Hmm lint is failing, I will review it!