Mailgun now offers multiple API endpoints.
The extension is currently hardcoded to only one.
Need to make use of this feature of the sdk
https://github.com/mailgun/mailgun-php/blob/master/src/Mailgun.php#L68
For users that select the non default api location for mailgun (such as EU).
Otherwise email sending will fail, if these API endpoints are used
Acceptance Criteria:
- It is possible to configure which Mailgun API is used by the extension (default remains the same)