Add a Postmark mail adapter so it can be configured as an outbound mailer

Summary: Depends on D19007. Ref T12677.

Test Plan: Used bin/mail send-test ... --mailer postmark to deliver some mail via Postmark.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12677

Differential Revision: https://secure.phabricator.com/D19009