Allow Metabase Cloud customers to specify a from email address

[cbalusek]

Is your feature request related to a problem? Please describe.
Metabase Cloud customers are unable to choose an email address to send emails from. As a result, emails are sent from a Metabase address.

Self-hosted Metabase customers are able to select a from address, as shown in the documentation here: https://www.metabase.com/docs/latest/configuring-metabase/email

Describe the solution you'd like

• Allow Metabase Cloud users to send emails from an email address using a custom domain
  • Metabase would need to provide DKIM and SPF TXT records for the customer to add to their DNS records to prevent messages getting flagged as spam

Describe alternatives you've considered

• Self-hosting is the only way around this currently.

How important is this feature to you?
It's a particular paint point for our Pro customers who use the whitelabeling / appearance features to customize their Metabase to look like their own branding, but emails still come from Metabase.