Fix schema authz check for non-CCloud message viewer instances

https://github.com/confluentinc/vscode/pull/654 started using `LocalEnvironment`s and introduced `DirectEnvironment`s, but there is still an old leftover assumption that local Kafka topics don't have an associated `environmentId` (which is false, since they have the `vscode-local-connection` ID):
https://github.com/confluentinc/vscode/blob/aca3bf54df9828ee98b5b409e5bb3d156a7af619/src/authz/schemaRegistry.ts#L29-L48

So when opening message viewer for a local/direct Kafka topic, we see these in the logs:
```
no Schema Registry in extension state matching CCloud topic's environment ID; assuming user can access (non-existent) schemas { environmentId: "vscode-local-connection" / }
```

We need to update this to correctly handle local/direct environments and migrate it to using the appropriate ResourceLoader instances.