feat: improved resource referencing and throw errors for invalid expressions

[astromechza]

This PR overhauls the placeholder referencing functionality and makes it more predictable and reliable in advance of a formal resource provisioning system.

1. ⚠️ Placeholder expressions that cannot be resolved now throw an error instead of returning empty string.

   So ${resources.unknown.nil} now throw an error.

2. All metadata keys and values are now available under ${metadata.<key>(.<key>)*}. nested maps can be indexed by key, strings are output as is, all other types are encoded as json.

   So ${metadata.name} -> hello-world. But ${metadata.info.author} might return {"email": "[email protected]"}.

3. The environment.default resource type is formalized so that ${resources.env.foo} is translated to ${FOO}.

   Keys are normalized, so ${resources.env.my-key} will translate to ${MY_KEY}.

4. Any other resource lookup falls back to the existing behavior of translating into a prefixed environment variable. eg: ${resources.db.foo} -> ${DB_FOO}. However now it does require that the resource db was actually declared.

   So ${resources.something.key.key} would fall back to ${SOMETHING_KEY_KEY}.
   In a future resource provisioning system, this behavior can be disabled or changed to opt-in.

5. volume.default resources are implemented as a static resource with no outputs - however the user must still provide the volume at this time. A future PR with resource provisioning will provide a default volume here.

Note that (3) will soon be replaced with a real resource provisioning system which will usually throw errors in this case unless the fallback behavior is opted-in.

Types 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)
• New chore (expected functionality to be implemented)

Checklist:

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I've signed off with an email address that matches the commit author.