[feat] ability to set default forwarder for the domain (e.g. upon new alias creation, use a default other than user's email)

[niftylettuce]

Ref: 📣 Your Help Request #1603392707262

[hp197]

+1 This is particular useful on password managers (read: Bitwarden) who don't have an option to set the forward to address and uses the email of the api user.

[chriskempson]

There is a feature request open for this on the Bitwarden Community Forums: https://community.bitwarden.com/t/forwardemail-net-allow-setting-recipients-in-service-config/76117