feat: create CI job to generate deployment Merge Requests [BB-4401]

This adds a way to run Tutor through the GitLab CI to create and update instance configurations. The task (triggered with a webhook) adds configurations from the request's body and creates a new MR, which in the future will be used for spawning a new instance.

Testing instructions

1. Fork the template repo.
2. Set DIGITALOCEAN_TOKEN, SPACES_ACCESS_KEY_ID, SPACES_SECRET_ACCESS_KEYin CI variables toDUMMY`.
3. Set a real token for GITLAB_TOKEN.
4. In your fork:
   1. Change the ref in .gitlab-ci.yml to agrendalath/bb-4401-generate_deployment_mr.
   2. Checkout the submodule on this branch.
   3. Commit and push your changes.
5. Follow this part of the README (you don't need a deployed cluster for this).
6. After pushing the request (from the docs), you should see a new job in your forked repo. It will open a new MR.