Support for config subentries

Config entries now have a new data type called “config subentry”. Config subentries are owned by a config entry and set up as part of async_setup_entry. Config subentries are created by config subentry flows and updated by config subentry reconfigure flows. This works similarly to how we create/edit config entries.

This makes it easier for integration authors to allow users to add, modify and remove pieces of configuration which share some common resource, for example a cloud account or an MQTT broker.

The architecture discussion gives more background.