Add support for Postmark and SendGrid APIs

[jadb]

Been asked a couple times why not just use SMTP. I thought it would be worth writing a little more in-depth explanation which should also keep me reminded of the real reasons that pushed me to code this for the current application I am working on.

While SMTP is great and PHP's native email function is super easy to get things started, there are other (and I believe better) solutions. The transports that are introduced in this PR are completely independent of each other and fill a single need in the general 'sending a transactional email' context/process.

Pros I find to using every transport type introduced:

API

• Performance: less interactions required by message (1 vs. 2+)
• Lower barrier: no need to have any SMTP requirements installed

DatabaseQueue

• Performance: sending of an email takes way more time than a database write (or any type of queuing)
• Fail-safe: database writes are transaction aware - if one operation fails, the entire transaction is rolled back
• Scalability: by modeling the email into queues, it becomes easier to scale in the future by re-implementing a different Queue transport (RabbitMQ, IronMQ, etc.) and leave the actual job to be handled by dedicated worker instances

Note: the DatabaseQueue'd job can then use the SmtpTransport (or any ApiTransport) to send the email.

Any comments/feedback/critic on the above explanation/reasoning or the committed code/implementations below are welcomed (I most probably directly linked you to this page for that exact reason 🎯 ).

[theraccoonbear]

@jadb just curious what the status of this pull request is. I am ramping up with a Cake3 project using SendGrid and am debating the approach to take.