feat: Add terraform modules for katib-{controller,db-manager,ui}

[orfeas-k]

Create a terraform/ directory for each of the charms that hosts their individual Terraform modules. It follows the structure proposed in this spec and it is based on what was done in canonical/argo-operators#198.

To test the modules:

• Clone the repository and switch this PR's branch.
• For each charm:
  • cd into its directory
  • First run tox -e tflint to ensure that linting is correct
  • Create a juju controller and a model name kubeflow
  • Run terraform apply -var "channel=latest/edge" -var "model_name=kubeflow" --auto-approve and wait until the charm is Active and Idle.

Ref #236
Ref #237
Ref #239

[mvlassis]

Note that the "terraform-apply" command results in a Blocked charm in the katib-controller, katib-db-manager, but that's expected behavior since the proper relations are missing.