adds two new plugins that use ansible-connection for persistence (#18572)

* adds new connection plugin `network_cli` which builds on paramiko
* adds new plugin `terminal` used for manipulating network_cli terminals
* adds new field to play_context `network_os` settable as ansible_network_os

This commit adds the plugins necesary to establish a persistent cli connection
to network devices of ssh.  It builds on the paramiko connection plugin
to create a shell environment that will persistent through ansible-connection.
The `newtork_cli` plugin then uses the network_os in the instance of
PlayContext to load the appropriate network OS environment plugin for
handling opening and closing of shells as well as privilege escalation.

lib/ansible/plugins/__init__.py

@@ -509,3 +509,11 @@ strategy_loader = PluginLoader(
     'strategy_plugins',
     required_base_class='StrategyBase',
 )
+
+terminal_loader = PluginLoader(
+    'TerminalModule',
+    'ansible.plugins.terminal',
+    'terminal_plugins',
+    'terminal_plugins'
+)
+