Each extension should be presented its own merged schema

[sipsma]

Right now there's just a global schema in cloak that gets presented to all the actions. The goal is for this to instead be per-action: the schema an action sees should consist of just its own direct dependencies. A few reasons this is desirable:

1. The cloak server can handle more than one client at a time
2. Less situations in which namespace conflicts arise; easier to add support for dealing with that
3. Actions that rely on different implementations of the same schema can coexist in the same DAG

[sipsma]

We also should consider using this as an initial, very primitive security mechanism for preventing arbitrary extensions from reading arbitrary host resources (like dirs and env vars). We can just never expose those APIs to extensions. We'll need more than than in the long term, but that's probably the simplest place to start.

[sipsma]

This is technically true now that dagger-in-dagger results in the creation of a new session. More adjustments will be needed when we revisit extensions but this issue isn't relevant anymore