Add control of Amcrest indicator light (#23986)

Enable feature by default but allow it to be disabled by "control_light: false" in config.

Get brand from camera instead of assuming Amcrest (since this works with other cameras, too.)

Retrieve RTSP URL in update method instead of in stream_source property and in handle_async_mjpeg_stream method.

Move amcrest imports from methods to global.

homeassistant/components/amcrest/binary_sensor.py

@@ -2,6 +2,8 @@
 from datetime import timedelta
 import logging
 
+from amcrest import AmcrestError
+
 from homeassistant.components.binary_sensor import (
     BinarySensorDevice, DEVICE_CLASS_MOTION)
 from homeassistant.const import CONF_NAME, CONF_BINARY_SENSORS
@@ -58,8 +60,6 @@ class AmcrestBinarySensor(BinarySensorDevice):
 
     def update(self):
         """Update entity."""
-        from amcrest import AmcrestError
-
         _LOGGER.debug('Pulling data from %s binary sensor', self._name)
 
         try: