* [ADD] base_external_system: Implement interface/adapter for external systems
* base_external_system: Fix OS model, add inherits, add validate
* base_external_system: Usability and private key pass
* base_external_system: Use contextmanager in adapter client
* base_external_system: Move contextmanager to interface
* base_external_system: Include contextmanager on adapter and system
* base_external_system: Unify client
* Use password widget for password field
* Add tests & security
* Fix lint
* Add plaintext note
