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/store-api.hh

@@ -89,6 +89,7 @@ const uint32_t exportMagic = 0x4558494e;
 
 
 enum BuildMode { bmNormal, bmRepair, bmCheck };
+enum TrustedFlag : bool { NotTrusted = false, Trusted = true };
 
 struct BuildResult;
 
@@ -815,6 +816,17 @@ public:
         return 0;
     };
 
+    /**
+     * @return/ whether store trusts *us*.
+     *
+     * `std::nullopt` means we do not know.
+     *
+     * @note This is the opposite of the StoreConfig::isTrusted
+     * store setting. That is about whether *we* trust the store.
+     */
+    virtual std::optional<TrustedFlag> isTrustedClient() = 0;
+
+
     virtual Path toRealPath(const Path & storePath)
     {
         return storePath;