patch(DPE-9339): MongoDB Encryption At Rest

[Gu1nness]

🏷️ Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Tooling and CI
• Dependencies upgrade or change
• Chores / refactoring

📝 Description

This is the implementation of MongoDB encryption at rest.
It uses Vault (Vault-k8s on kubernetes) as a backend to store the encryption keys.
It uses a config option to enable encryption at rest at boot time.
It uses an action to rotate the master key.

🧪 Manual testing steps

🌞 Sunny test

1. juju deploy <my-app> --config enable-encryption-at-rest=true
2. follow [the tutorial](https://canonical-vault-charms.readthedocs-hosted.com/en/latest/tutorial/getting_started_k8s/) to deploy and unseal vault/vault-k8s
3. juju integrate <my-app>:vault-kv vault:vault-kv
4. Profit (you can check the startup options of mongodb to ensure that it contains enableEncryption: True)
5. juju run <my-app>/<a-unit> rotate-encryption-master-key
6. Check in log file that it contains `Rotated master encryption key`.

🌧️ Rainy test:

1. juju deploy <my-app> --config enable-encryption-at-rest=false (the default)
2. follow [the tutorial](https://canonical-vault-charms.readthedocs-hosted.com/en/latest/tutorial/getting_started_k8s/) to deploy and unseal vault/vault-k8s
3. juju integrate <my-app>:vault-kv vault:vault-kv
4. It goes to blocked BUT does not prevent the charm from operating.

🔬 Automated testing steps

Positive checks that:

• Deployment goes to blocked until integrated with vault
• Integration restarts with correct options
• Rotation works
• Removing the relation goes to blocked.

Negative checks that:

• Deploying with encryption disabled and integrating with vault goes to blocked
• Trying to rotate master key in that scenario fails.

✅ Checklist

• My code follows the code style of this project.
• I have added or updated any relevant documentation.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[patriciareinoso]

extremely quick pass over the code