MM-40813: Add config interface (#19571)

We create the basic boilerplate to pass services
to each product. The basic idea is to have a map
containing service names and the services.

The server will pass all services to all products.
But it is on the product, to choose what services
they will actually consume.

Each product will cast the received service to
an interface with the methods that product requires.

https://mattermost.atlassian.net/browse/MM-40813

```release-note
NONE
```

app/channels.go

@@ -4,6 +4,7 @@
 package app
 
 import (
+	"fmt"
 	"runtime"
 	"strings"
 	"sync"
@@ -20,9 +21,26 @@ import (
 	"github.com/pkg/errors"
 )
 
+// configSvc is a consumer interface to work
+// with any config related task with the server.
+type configSvc interface {
+	Config() *model.Config
+	AddConfigListener(listener func(*model.Config, *model.Config)) string
+	RemoveConfigListener(id string)
+	UpdateConfig(f func(*model.Config))
+	SaveConfig(newCfg *model.Config, sendConfigChangeClusterMessage bool) (*model.Config, *model.Config, *model.AppError)
+}
+
+// namer is an interface which enforces that
+// all services can return their names.
+type namer interface {
+	Name() ServiceKey
+}
+
 // Channels contains all channels related state.
 type Channels struct {
-	srv *Server
+	srv    *Server
+	cfgSvc configSvc
 
 	postActionCookieSecret []byte
 
@@ -65,17 +83,44 @@ type Channels struct {
 }
 
 func init() {
-	RegisterProduct("channels", func(s *Server) (Product, error) {
-		return NewChannels(s)
+	RegisterProduct("channels", func(s *Server, services map[ServiceKey]interface{}) (Product, error) {
+		return NewChannels(s, services)
 	})
 }
 
-func NewChannels(s *Server) (*Channels, error) {
+func NewChannels(s *Server, services map[ServiceKey]interface{}) (*Channels, error) {
+
 	ch := &Channels{
 		srv:           s,
 		imageProxy:    imageproxy.MakeImageProxy(s, s.httpService, s.Log),
 		uploadLockMap: map[string]bool{},
 	}
+
+	// To get another service:
+	// 1. Prepare the service interface
+	// 2. Add the field to *Channels
+	// 3. Add the service key to the slice.
+	// 4. Add a new case in the switch statement.
+	requiredServices := []ServiceKey{ConfigKey}
+	for _, svcKey := range requiredServices {
+		svc, ok := services[svcKey]
+		if !ok {
+			return nil, fmt.Errorf("Service %s not passed", svcKey)
+		}
+		switch svcKey {
+		// Keep adding more services here
+		case ConfigKey:
+			cfgSvc, ok := svc.(configSvc)
+			if !ok {
+				return nil, errors.New("Config service did not satisfy ConfigSvc interface")
+			}
+			_, ok = svc.(namer)
+			if !ok {
+				return nil, errors.New("Config service does not contain Name method")
+			}
+			ch.cfgSvc = cfgSvc
+		}
+	}
 	// We are passing a partially filled Channels struct so that the enterprise
 	// methods can have access to app methods.
 	// Otherwise, passing server would mean it has to call s.Channels(),
@@ -119,7 +164,7 @@ func NewChannels(s *Server) (*Channels, error) {
 func (ch *Channels) Start() error {
 	// Start plugins
 	ctx := request.EmptyContext()
-	ch.initPlugins(ctx, *ch.srv.Config().PluginSettings.Directory, *ch.srv.Config().PluginSettings.ClientDirectory)
+	ch.initPlugins(ctx, *ch.cfgSvc.Config().PluginSettings.Directory, *ch.cfgSvc.Config().PluginSettings.ClientDirectory)
 
 	ch.AddConfigListener(func(prevCfg, cfg *model.Config) {
 		// We compute the difference between configs
@@ -142,7 +187,7 @@ func (ch *Channels) Start() error {
 		// Do only if some plugin related settings has changed.
 		if hasDiff {
 			if *cfg.PluginSettings.Enable {
-				ch.initPlugins(ctx, *cfg.PluginSettings.Directory, *ch.srv.Config().PluginSettings.ClientDirectory)
+				ch.initPlugins(ctx, *cfg.PluginSettings.Directory, *ch.cfgSvc.Config().PluginSettings.ClientDirectory)
 			} else {
 				ch.ShutDownPlugins()
 			}