Support Custom Email Domain Sender on Authgear Cloud

I think a project for the above would be useful / expected by many customers.

Approaches:

1. Consider switch back to use Postmark instead (or other reputation email delivery services with domains API)
2. Design and implement a Portal UI that's feature flag enabled for our SaaS platform only, to:
1. Change email sender
2. Add email domains / Verify DKIM, SPF etc.
3. Connect Postmark API for such features: [https://postmarkapp.com/developer/api/domains-api#create-domain](https://postmarkapp.com/developer/api/domains-api#create-domain)

The issue names no files, tests, or entry points. Start by comparing the proposed Postmark migration with the feature-flagged Portal UI and domain-verification approaches, then define the scope and acceptance criteria. Done requires an agreed design covering sender changes, domain verification, and the selected delivery-service integration.

Written by the indexing model from the issue text.