feat: CI

[pol-defont-reaulx]

Hello all, here is a first draft of OCMLF's CI.
Give me your feedbacks on this.

Issue

My PR resolves #27

Description

• add "make validate-terraform" to validate IaC without a backend
• people can give a project id instead of a numerical choice (to be able to specify the right project during the test)
• add a YAML file that will be used by Cloud Build

To do manually to be able to test this

• Create a project that will manage ocmlf's CI
• Create a SA with roles/resourcemanager.projectCreator to be able to create a new project with this SA
• Give permissions to Cloud Build's SA to impersonate the created SA
• Create a Docker image with GCP SDK and Terraform and push it to GCP
• Create Cloud Build triggers for GitHub PRs on this project

Questions

1. To finish the CI we'll need to do some steps manually (see previous section), but I could also make these steps with Terraform even if they'll need to be done only one time. What do you think is the best here?
2. I wasn't able to find an image with both gcp sdk and terraform on it, do you know one that could be used instead of creating and storing our own?
3. We could also add some tests using MLFlow's API at the end to see if all is working good, what do you think?