Add support for learning new commands (#23888)

* Add support for learning new commands

This update creates a generic service in the 'remote' component to enable remote control platforms to learn new commands.

* Update __init__.py with the proposed changes

- Add 'supported_features' property and a constant related to the 'learn_command' functionality.
- Redefine 'async_learn_command' function as a coroutine.

* Update __init__.py

* Fix assertion error

Adding the 'supported_features' attribute generated an assertion error on the 'Demo Remote' platform. This update fixes this.

* Fix duplicated 'hass' object

This update fixes a typo that occurred at the last update.

tests/components/demo/test_remote.py

@@ -48,5 +48,8 @@ class TestDemoRemote(unittest.TestCase):
         common.send_command(self.hass, 'test', entity_id=ENTITY_ID)
         self.hass.block_till_done()
         state = self.hass.states.get(ENTITY_ID)
-        assert state.attributes == \
-            {'friendly_name': 'Remote One', 'last_command_sent': 'test'}
+        assert state.attributes == {
+            'friendly_name': 'Remote One',
+            'last_command_sent': 'test',
+            'supported_features': 0
+        }