Terraform provider backed Juju model setup + associated charmed operators for API and backend

[mz2]

Introduces K8s backed deployment methodology using charms and terraform that builds out a Juju model with the application:

• Docker image publishing to ghcr.io/canonical/test-observer/api and ghcr.io/canonical/test-observer/frontend respectively.
• A K8s charm for the backend.
• A K8s charm for the frontend.
• Terraform configuration that sets up a model with the entire system, with database, ingress (optionally SSL certs) included
• README updates to cover all this.
• Moves source code from src to test_observer so that import paths work and poetry build actually works (building a wheel was necessary in order to get dynamic versioning working).

Tested locally by multiple passes of setting up and tearing down the Juju model, reviewer should do the same following the steps in the README.

[nadzyah]

Some other minor comments. Also, please, make sure your changes don't rollback #14 PR

[omar-selo]

Note that pytest is failing. Imports need updating in tests directory

[mz2]

Note that pytest is failing. Imports need updating in tests directory

Yes, all sorts of things break I imagine. As I keep stressing, CI needs improvements now, obviously we cannot collaborate effectively without knowing when linting, formatting, testing, building different artefacts breaks. That goes to frontend, backend, charms that operate them. All of this is best done in subsequent PRs because this PR introduces changes that will conflict will anything you would do with main now.

[mz2]

Some other minor comments. Also, please, make sure your changes don't rollback #14 PR

To explain this the Nth time for posterity, the conflicts between main and this branch are down to there having been a number of PRs merged whilst this was in motion which move files under src around, and this branch moves src to a directory called test_observer to make poetry build successfully complete. Since this lead to import dependencies changing, things got messy with lots of merge conflicts, and this also motivates quickly getting this merged before action on other PRs, and focusing for a few days on CI improvements that make it clear when linting, formatting, testing, building kind of actions of any of the build outputs in this repo break... in follow-up PRs.

[nadzyah]

All of my concerns were addressed and resolved. Thanks for these changes!