Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/google.golang.org/grpc/encoding/internal/internal.go

@@ -0,0 +1,28 @@
+/*
+ *
+ * Copyright 2025 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Package internal contains code internal to the encoding package.
+package internal
+
+// RegisterCompressorForTesting registers a compressor in the global compressor
+// registry. It returns a cleanup function that should be called at the end
+// of the test to unregister the compressor.
+//
+// This prevents compressors registered in one test from appearing in the
+// encoding headers of subsequent tests.
+var RegisterCompressorForTesting any // func RegisterCompressor(c Compressor) func()