TLS Integration SMTP support

[jake-walker]

What does this PR do?

This PR adds support for SMTP to the TLS integration. It adds functionality for a plain-text SMTP connection to be upgraded to TLS for the TLS integration to be able to check the certificate of the server.

I have added new code for upgrading the connection in a similar fashion to the existing MySQL and Postgres implementations. In addition, I have added a new test and container to match the existing implementations.

Motivation

This was needed at my workplace for monitoring SMTP TLS certificates.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.01%. Comparing base (a28034c) to head (2f55c73).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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Review from janine-c is dismissed. Related teams and files:

• documentation
  • tls/datadog_checks/tls/data/conf.yaml.example