Represent coder_app instances in the agents.

[defelmnq]

Description

With devcontainers, we want to introduce a new way to inject coder_app inside containers, not only using terraform, but also devcontainers.json.

As of now, we do not have a clear representation of what is a coder_app inside the workspaces. It will be useful to have it, in a unified way, so we can merge all the coder_app.
It will also simplify the communication with coderd as all the coder_app will be represented the same way.

• Replace rbac.ResourceSystem usage in dbauthz.InsertWorkspaceApp coder#18210