[ADD] base_external_system: Implement interface/adapter (#993)

* [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

base_external_system/README.rst

@@ -0,0 +1,96 @@
+.. image:: https://img.shields.io/badge/license-LGPL--3-blue.svg
+   :target: http://www.gnu.org/licenses/lgpl.html
+   :alt: License: LGPL-3
+
+======================
+Base - External System
+======================
+
+This module provides an interface/adapter mechanism for the definition of remote
+systems.
+
+Note that this module stores everything in plain text. In the interest of security,
+it is recommended you use another module (such as `keychain` or `red_october` to
+encrypt things like the password and private key). This is not done here in order
+to not force a specific security method.
+
+Implementation
+==============
+
+The credentials for systems are stored in the ``external.system`` model, and are to
+be configured by the user. This model is the unified interface for the underlying
+adapters.
+
+Using the Interface
+-------------------
+
+Given an ``external.system`` singleton called ``external_system``, you would do the
+following to get the underlying system client:
+
+.. code-block:: python
+
+   with external_system.client() as client:
+       client.do_something()
+
+The client will be destroyed once the context has completed. Destruction takes place
+in the adapter's ``external_destroy_client`` method.
+
+The only unified aspect of this interface is the client connection itself. Other more
+opinionated interface/adapter mechanisms can be implemented in other modules, such as
+the file system interface in `OCA/server-tools/external_file_location
+<https://github.com/OCA/server-tools/tree/9.0/external_file_location>`_.
+
+Creating an Adapter
+-------------------
+
+Modules looking to add an external system adapter should inherit the
+``external.system.adapter`` model and override the following methods:
+
+* ``external_get_client``: Returns a usable client for the system
+* ``external_destroy_client``: Destroy the connection, if applicable. Does not need
+  to be defined if the connection destroys itself.
+
+Configuration
+=============
+
+Configure external systems in Settings => Technical => External Systems
+
+.. image:: https://odoo-community.org/website/image/ir.attachment/5784_f2813bd/datas
+   :alt: Try me on Runbot
+   :target: https://runbot.odoo-community.org/runbot/149/10.0
+
+Bug Tracker
+===========
+
+Bugs are tracked on `GitHub Issues
+<https://github.com/OCA/server-tools/issues>`_. In case of trouble, please
+check there if your issue has already been reported. If you spotted it first,
+help us smash it by providing detailed and welcomed feedback.
+
+Credits
+=======
+
+Images
+------
+
+* Odoo Community Association: `Icon <https://github.com/OCA/maintainer-tools/blob/master/template/module/static/description/icon.svg>`_.
+
+Contributors
+------------
+
+* Dave Lasley <dave@laslabs.com>
+
+Maintainer
+----------
+
+.. image:: https://odoo-community.org/logo.png
+   :alt: Odoo Community Association
+   :target: https://odoo-community.org
+
+This module is maintained by the OCA.
+
+OCA, or the Odoo Community Association, is a nonprofit organization whose
+mission is to support the collaborative development of Odoo features and
+promote its widespread use.
+
+To contribute to this module, please visit https://odoo-community.org.