* [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

* In cases of deep inheritance, it may be required to create an adapter directly. Add an override in the create via the env context to support this.
* Add default system type if creating from an interface
* Fix interface assignment during creation in adapter

* Perform standard v11 migration

Testing fingerprint, with ignore_finger_print set to False, caused an error, because it would first set
ignore_fingerprint to False, then validate, never getting to actually setting the fingerprint, causing the
following stacktrace:
Traceback (most recent call last):

  File "/home/travis/build/OCA/server-backend/base_external_system/tests/test_external_system.py", line 28, in test_check_fingerprint_allowed

    self.record.write({"ignore_fingerprint": False, "fingerprint": "Data"})

  File "/home/travis/odoo-13.0/odoo/models.py", line 3573, in write

    fields[0].determine_inverse(real_recs)

  File "/home/travis/odoo-13.0/odoo/fields.py", line 1122, in determine_inverse

    self.inverse(records)

  File "/home/travis/odoo-13.0/odoo/fields.py", line 594, in _inverse_related

    target[field.name] = record_value[record]

  File "/home/travis/odoo-13.0/odoo/models.py", line 5629, in __setitem__

    return self._fields[key].__set__(self, value)

  File "/home/travis/odoo-13.0/odoo/fields.py", line 1087, in __set__

    records.write({self.name: write_value})

  File "/home/travis/odoo-13.0/odoo/models.py", line 3568, in write

    real_recs._validate_fields(set(vals) - set(inverse_fields))

  File "/home/travis/odoo-13.0/odoo/models.py", line 1176, in _validate_fields

    check(self)