Ability to specify multiple SMTP servers for email alerting

Currently, it is only possible to specify one SMTP server for outgoing email alerts for the graylog server.

For example, if I want to send email alerts to a gmail account AND to a local corporate email exchange, it is not possible due to the transport_email_hostname variable only being able to be defined once in /etc/graylog/server/server.conf

It would be better to define these variables on a notifcation by notifcation basis in the GUI itself. (Similar to how zabbix handles several media types, which could include several smtp servers for multiple email addresses)

* Graylog Version: 4.0.14+224b726
* Elasticsearch Version: 6.8.20
* MongoDB Version: 4.4.10
* Operating System: Debian 11.0.13 on Linux 5.10.0-9-amd64
* Browser version: linux Firefox 96.0