MM-43145: channels loader (#19957)

Add channels dataloader

https://mattermost.atlassian.net/browse/MM-43145

```release-note
NONE
```

api4/graphql.go

@@ -62,10 +62,13 @@ func (api *API) InitGraphQL() error {
 type ctxKey int
 
 const (
-	webCtx         ctxKey = 0
-	rolesLoaderCtx ctxKey = 1
+	webCtx            ctxKey = 0
+	rolesLoaderCtx    ctxKey = 1
+	channelsLoaderCtx ctxKey = 2
 )
 
+const loaderBatchCapacity = 200
+
 func (api *API) graphQL(c *Context, w http.ResponseWriter, r *http.Request) {
 	var response *graphql.Response
 	defer func() {
@@ -98,9 +101,12 @@ func (api *API) graphQL(c *Context, w http.ResponseWriter, r *http.Request) {
 	reqCtx := r.Context()
 	reqCtx = context.WithValue(reqCtx, webCtx, c)
 
-	rolesLoader := dataloader.NewBatchedLoader(graphQLRolesLoader, dataloader.WithBatchCapacity(200))
+	rolesLoader := dataloader.NewBatchedLoader(graphQLRolesLoader, dataloader.WithBatchCapacity(loaderBatchCapacity))
 	reqCtx = context.WithValue(reqCtx, rolesLoaderCtx, rolesLoader)
 
+	channelsLoader := dataloader.NewBatchedLoader(graphQLChannelsLoader, dataloader.WithBatchCapacity(loaderBatchCapacity))
+	reqCtx = context.WithValue(reqCtx, channelsLoaderCtx, channelsLoader)
+
 	response = api.schema.Exec(reqCtx,
 		params.Query,
 		params.OperationName,