Public API for Sender ID, Reply ID Listing & Test Email Send

Key highlights:
- List Sender IDs via API to retrieve all verified sender addresses in your account
- List Reply IDs to fetch configured reply-to addresses programmatically
- Send test emails using pre-saved test groups directly through the API
- Enables seamless integration with custom workflows and automation pipelines
This update makes it easier for technical teams to build and automate their email sending processes without relying on manual steps in the Mailercloud interface.