Add an applicationset to automatically deploy the swh next version in staging

Deploy a new environment in // of staging with the new components It's a wip as it should be done only if there is no migrations to apply

Related to

MR description: It's a first iteration on the subject because only deploying the pod is not enough. We should launch tests on the deployment to validate the stability.

It raises some concerns relative to the automatic deployment:

• The resources allocated to the staging cluster will need to be increased to be able to handled the new resources that it will deploy
• The swh-cassandra secrets need to be duplicated to the swh-cassandra-next-version namespace
• In term if security, we need to be sure of what is pushed in the merge request (for example if somebody manually creates a swh-next-version mr) to avoid deploying uncontrolled deployments in the staging environnement

For info, the generator documentation: https://argo-cd.readthedocs.io/en/stable/operator-manual/applicationset/Generators-Pull-Request/