Add available Nextcloud update information

[mafgh]

I'd like to monitor for an available update with an external tool.

Please consider adding something like this:

'available' => json_decode( $this->config->getAppValue( 'core', 'lastupdateResult' ) ),
'lastupdatedat' => $this->config->getAppValue('core', 'lastupdatedat'),

for example below data/nextcloud/system/update.