feat(chat): add /guilds/{id} and /guilds/@me endpoints

Foxchat.Chat/Middleware/ServerAuthenticationMiddleware.cs

@@ -76,4 +76,11 @@ public static class HttpContextExtensions
 
         return ((IdentityInstance, SignatureData))obj!;
     }
+
+    public static (IdentityInstance, SignatureData, string) GetSignatureWithUser(this HttpContext ctx)
+    {
+        var (instance, sig) = ctx.GetSignatureOrThrow();
+        if (sig.UserId == null) throw new ApiError.IncomingFederationError("This endpoint requires a user ID.");
+        return (instance, sig, sig.UserId);
+    }
 }