Investigate using symfony/mailgun-mailer transport

[JamesDPC]

The current process for sending email via the Mailgun API involves registering a Mailer (MailgunMailer) and connecting to the mailgun php sdk via a connection client (Connector\Message).

The Mailer handles stand to/from/subject and also the passing custom parameters from project code to the Mailgun API. The connector can make some decisions about how and when the message should be sent based on those parameters + project configuration. Sending out-of-band as a job, for instance.

Silverstripe framework >= 4.9.0 moved to swiftmailer v6 support BUT swiftmailer/swiftmailer has been archived with a note to move to symfony/mailer. There is a related open issue: silverstripe/silverstripe-framework#10097

symfony/mailer provides a Mailgun transport: https://github.com/symfony/mailgun-mailer which we could investigate using as it could reduce duplication of effort, provided the transport supports what can be done currently.

That said, any change would need to support:

• sending later via a job, set via configuration
• all of the Mailgun API parameters currently supported (options, variables, recipient variables, ... https://documentation.mailgun.com/en/latest/api-sending.html#sending)
• webhook support
• current configuration options: always set sender, always from, testmode, api region
• integration with or modification of https://github.com/nswdpc/silverstripe-mailgun-sync/tree/feat-taggable

The end result should be:

1. Create an email in the standard way (Email::create)
2. Assign parameters at send time, via extension or via configuration
3. Send the email via the configured transport, now or later.

If BC could be ensured, it could be tagged as v1.x