Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

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

vendor/google.golang.org/grpc/internal/balancer/weight/weight.go

@@ -0,0 +1,66 @@
+/*
+ *
+ * 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 weight contains utilities to manage endpoint weights. Weights are
+// used by LB policies such as ringhash to distribute load across multiple
+// endpoints.
+package weight
+
+import (
+	"fmt"
+
+	"google.golang.org/grpc/resolver"
+)
+
+// attributeKey is the type used as the key to store EndpointInfo in the
+// Attributes field of resolver.Endpoint.
+type attributeKey struct{}
+
+// EndpointInfo will be stored in the Attributes field of Endpoints in order to
+// use the ringhash balancer.
+type EndpointInfo struct {
+	Weight uint32
+}
+
+// Equal allows the values to be compared by Attributes.Equal.
+func (a EndpointInfo) Equal(o any) bool {
+	oa, ok := o.(EndpointInfo)
+	return ok && oa.Weight == a.Weight
+}
+
+// Set returns a copy of endpoint in which the Attributes field is updated with
+// EndpointInfo.
+func Set(endpoint resolver.Endpoint, epInfo EndpointInfo) resolver.Endpoint {
+	endpoint.Attributes = endpoint.Attributes.WithValue(attributeKey{}, epInfo)
+	return endpoint
+}
+
+// String returns a human-readable representation of EndpointInfo.
+// This method is intended for logging, testing, and debugging purposes only.
+// Do not rely on the output format, as it is not guaranteed to remain stable.
+func (a EndpointInfo) String() string {
+	return fmt.Sprintf("Weight: %d", a.Weight)
+}
+
+// FromEndpoint returns the EndpointInfo stored in the Attributes field of an
+// endpoint. It returns an empty EndpointInfo if attribute is not found.
+func FromEndpoint(endpoint resolver.Endpoint) EndpointInfo {
+	v := endpoint.Attributes.Value(attributeKey{})
+	ei, _ := v.(EndpointInfo)
+	return ei
+}