patch(DPE-9456): Send CA certificate to clients

[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

Send CA certificate to client
Also does it on new certificates, and on update of clients.

🧪 Manual testing steps

After building the charm, run

1. juju deploy <my-app>
2. juju deploy self-signed-certificates
3. juju deploy data-integrator --config database-name="testdb"
4. juju integrate <my-app>:client-certificates self-signed-certificates
5. juju integrate <my-app> data-integrator
6. juju run data-integrator/0 get-credentials

This should display the certificate in the field tls-ca and a flag to True in the field tls.

🔬 Automated testing steps

One test added on integration testing step to ensure that the certificate is in the databag.

✅ 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.