feat: Adding Terraform module

[Gmerold]

Context:
Telco team is trying to build Terraform modules for the Charmed 5G, using the Juju Terraform provider. Main reason for having Terraform modules is that, unlike Juju bundles, they support day 2 operations.

This PR adds a Terraform module for the Prometheus charm.
The module uses Terraform Juju provider and allows deploying Alertmanager using Terraform.

All details regarding module's architecture are available in terraform/README.md.

In the next step, this base module will be used by the cos-lite Terraform module (prototype here).

[simskij]

So, we had a brief discussion about this yesterday, and we haven't really agreed on whether we want the modules scattered across the repos, or if we want all of them in the cos-lite-bundle repo. Let's park this PR until we've had a chance to decide.

[Gmerold]

So, we had a brief discussion about this yesterday, and we haven't really agreed on whether we want the modules scattered across the repos, or if we want all of them in the cos-lite-bundle repo. Let's park this PR until we've had a chance to decide.

Sure. In case you wanted something to compare against, an example of individual modules co-located with the bundle is in https://github.com/Gmerold/terraform-juju-cos-lite-k8s/tree/cos-lite-module