[Config] Add Group.clear_raw method (#2178)

Adds a `clear_raw` method to Group objects, similar to the existing `get_raw` and `set_raw` methods.
Documentation included.

tests/core/test_config.py

@@ -224,6 +224,15 @@ async def test_set_dynamic_attr(config):
     assert await config.foobar() is True
 
 
+@pytest.mark.asyncio
+async def test_clear_dynamic_attr(config):
+    await config.foo.set(True)
+    await config.clear_raw("foo")
+
+    with pytest.raises(KeyError):
+        await config.get_raw("foo")
+
+
 @pytest.mark.asyncio
 async def test_get_dynamic_attr(config):
     assert await config.get_raw("foobaz", default=True) is True