feat: implement terraform modules and infrastructure example

[gabor-boros]

This PR implements terraform modules for DigitalOcean that allows anyone to create a production-grade infrastructure for hosting Open edX instances using Tutor and Harmony.

The PR includes a reference implementation as a usage example.

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[openedx-webhooks]

Thanks for the pull request, @gabor-boros!

What's next?

Please work through the following steps to get your changes ready for engineering review:

🔘 Get product approval

If you haven't already, check this list to see if your contribution needs to go through the product review process.

• If it does, you'll need to submit a product proposal for your contribution, and have it reviewed by the Product Working Group.
  • This process (including the steps you'll need to take) is documented here.
• If it doesn't, simply proceed with the next step.

🔘 Provide context

To help your reviewers and other members of the community understand the purpose and larger context of your changes, feel free to add as much of the following information to the PR description as you can:

• Dependencies

  This PR must be merged before / after / at the same time as ...

• Blockers

  This PR is waiting for OEP-1234 to be accepted.

• Timeline information

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• Partner information

  This is for a course on edx.org.

• Supporting documentation
• Relevant Open edX discussion forum threads

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[gabor-boros]

@bradenmacdonald could you please take another review and approve if you agree with the changes?

[bradenmacdonald]

@gabor-boros LGTM. Thanks!

[gabor-boros]

@bradenmacdonald Before I merge this, I would like to highlight that this implementation has the exact same "features" that you highlighted as weaknesses in the AWS infra PR