Add server/cache support to gaeconfig.

luci_config/appengine/format/context.go

+// Copyright 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package format
+
+import (
+        "github.com/luci/luci-go/luci_config/server/cfgclient"
+
+        "golang.org/x/net/context"
+)
+
+var formatRegistryKey = "github.com/luci/luci-go/luci_config/appengine/format:FormatRegistry"
+
+// WithRegistry returns a Context with the supplied FormatterRegistry embedded
+// into it.
+func WithRegistry(c context.Context, fr *cfgclient.FormatterRegistry) context.Context {
+        return context.WithValue(c, &formatRegistryKey, fr)
+}
+
+// GetRegistry is used to retrieve the config service format registry from the
+// supplied Context.
+func GetRegistry(c context.Context) *cfgclient.FormatterRegistry {
+        return c.Value(&formatRegistryKey).(*cfgclient.FormatterRegistry)
+}