Add Store::isTrustedClient()

This function returns true or false depending on whether the Nix client
is trusted or not. Mostly relevant when speaking to a remote store with
a daemon.

We include this information in `nix ping store` and `nix doctor`

Co-Authored-By: John Ericson <John.Ericson@Obsidian.Systems>

src/libstore/dummy-store.cc

@@ -39,6 +39,14 @@ struct DummyStore : public virtual DummyStoreConfig, public virtual Store
         callback(nullptr);
     }
 
+    /**
+     * The dummy store is incapable of *not* trusting! :)
+     */
+    virtual std::optional<TrustedFlag> isTrustedClient() override
+    {
+        return Trusted;
+    }
+
     static std::set<std::string> uriSchemes() {
         return {"dummy"};
     }