Add API Endpoints for domain settings and sender information

We are working on expand the domain API with a few missing endpoints related to domain configuration and sender management.

The main request is to add API support for:

• updating domain settings

• retrieving current sender information

• creating or updating sender information

At the moment, these actions require manual work in the app, which makes automation harder. Adding these endpoints will help teams manage domains fully through API and keep their setup in sync with internal tools, scripts, or onboarding flows.