Update to the Mailgun Public API domain, including ability to generate an API key programmatically

Admin users can now programmatically create API keys via an API Keys API endpoint. This also includes the ability for an admin user to create a sending/domain API key using a primary account API key. Additionally, we made public our SMTP Credentials API and IP Allowlist (Whitelist) API. For more information on public API endpoints and access levels per role, please visit https://documentation.mailgun.com/docs/mailgun/user-manual/mg_security/#rbac-api-key-permissions-based-on-role.