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/local-store.hh

@@ -204,6 +204,8 @@ public:
 
     unsigned int getProtocol() override;
 
+    std::optional<TrustedFlag> isTrustedClient() override;
+
     void vacuumDB();
 
     void repairPath(const StorePath & path) override;