Platform agnostic net_system module (#24953)

* Platform agnostic net_system module

Also refactor the action network plugins for better code re-use

Still more refactoring to do once the connection plugin work is complete

* Replace importlib for imp

importlib is not available on 2.6, so we need to stick to imp

* Load action plugin via module metadata

* Better error message if no implementation is found

Now the plugin will show the module name and the network OS in the
error message

* Fix typo on documentation author line

* Fix pep8 issues

* Add missing options key on doc string and stringify version

* Return None in case module has no metadata

* Read module metadata only if it's a python module

Check for module suffix, if it's .py then read metadata.
Otherwise this fails on non-python modules, like Windows PS for example.

* Read metadata variable only if it's a python module

Fix referencing a variable before assignment

* Add action_handler to validate_modules metadata schema

* Pull metadata with plugin_docs get_docstring

Using load_source from PluginLoader is troublesome, it is not guaranteed
a module may be importable at the controller, e.g. if a module depends
on module_utils functions it won't work, because module_utils is not
in the sys path.
Rather than putting that module dependencies introspection, just
use plain parsing like plugin_docs get_docstring does as we only care
about reading ANSIBLE_METADATA.

* Add platform agnostic group of groups for integration tests

This will be the target for platform agnostic integration tests.

* Add integration tests for net_system

* Switch to action plugin inheritance from metadata driven action handler

As the metadata action driven action handler work is being worked on
on its standalone proposal+PR, let's just go back to have one
action handler per platform agnostic module.
Those action plugins will inherit from net_base.

* Add blank line to fix pep8

* Add aliases file to net_system integration test

This will avoid CI failure

* Fix integration tests for net_system

* Give more precedence to task network_os over inventory network_os

test/integration/targets/net_system/tasks/cli.yaml

@@ -0,0 +1,16 @@
+---
+- name: collect all cli test cases
+  find:
+    paths: "{{ role_path }}/tests/cli"
+    patterns: "{{ testcase }}.yaml"
+  register: test_cases
+  delegate_to: localhost
+
+- name: set test_items
+  set_fact: test_items="{{ test_cases.files | map(attribute='path') | list }}"
+
+- name: run test case
+  include: "{{ test_case_to_run }}"
+  with_items: "{{ test_items }}"
+  loop_control:
+    loop_var: test_case_to_run

test/integration/targets/net_system/tasks/main.yaml

@@ -0,0 +1,2 @@
+---
+- { include: cli.yaml, tags: ['cli'] }