[MVC] Create PoC for storing secrets for groups/projects under separate mounts

@fabiopitino brought up a great point that it is best to design our architecture around the best possible way to enforce isolation across groups/projects/tenants secrets.

In this issue we want to explore and create a PoC for storing secrets per tenant (group/project) under their own secrets mount. Let's take note of all the benefits that we can get by doing this and make sure to update the blueprint with the necessary ADRs.