Support SMTP connection pooling when sending emails

What does this MR do?

This adds a new ActionMailer delivery method for sending emails using an SMTP connection pool. This is implemented as a gem under vendor/ so that we can easily extract this in the future.

Related to #230717 (closed)

Testing locally

You can test this by following the instructions in config/initializers/smtp_settings.rb.sample

You may need to set action_mailer.perform_deliveries = true because this is set to false in development.

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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