New helpers and best practises for reauth and reconfigure flows

New helper methods have been added to the ConfigFlow to facilitate management of reauth and reconfigure flows:

• self._get_reauth_entry() and self._get_reconfigure_entry() give access at any time to the corresponding config entry
  • these should be used over self.hass.config_entries.async_get_entry(self.context["entry_id"])
  • the config entry should be requested when needed (local variable, once per step) and not cached as class attributes
  • if the steps are shared with discovery or user flows, self.source should be checked against SOURCE_REAUTH and SOURCE_RECONFIGURE before accessing the entry
• self._abort_if_unique_id_mismatch allows you to abort if the unique_id does not match the unique_id of the config entry to reauthenticate or reconfigure
  • this should be used after a call to self.async_set_unique_id
  • if the steps are shared with discovery or user flows, self.source should be checked against SOURCE_REAUTH and SOURCE_RECONFIGURE
  • other sources should continue to use self._abort_if_unique_id_configured
• self.async_update_reload_and_abort has been adjusted to update the default message for reconfigure flows
  • the new message reconfigure_successful must be present in strings.json
• self.async_update_reload_and_abort has a new argument data_updates to merge the data updates with the pre-existing data
  • this is preferred over the data argument, as it reduces the risk of data loss if the schema is updated

More details can be found in the reconfigure and reauthentication documentation.