feat: integration tests CI

[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 changes
• Dependencies upgrade or change

📝 Description

Introduction of a CI workflow that listens on comments on PRs and opens PR on the remote repositories to trigger the CI.
It then posts the PR URL so we can fetch the results.

This is run on comment on a PR, following this syntax:

• /test to run on 4 charms.
• /test/<mongod | mongos>/<vm | k8s> to run on a specific charm.
• /test/*/<vm | k8s> to run for both charms on a specific substrate.

📑 Motivation and Context

This is the final step required for the single kernel charm.
A way of easily running the CI tests on the remote charms.

🧪 How Has This Been Tested?

This needs to be tested.

✅ Checklist

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

[MiaAltieri]

nice

[MiaAltieri]

you are a rockstar