MM-45956: Optimize FileInfo stats query (#22603)

* MM-45956: Optimize FileInfo stats query

We Denormalize Post.ChannelId on FileInfo.ChannelId

```release-note
The file info stats query is now optimized by denormalizing the channelID column into the table itself. This will speed up the query to get the file count for a channel on clicking the RHS.

Migration times:
On a MySQL 8.0.31 DB with
1405 rows in FileInfo and 11M posts, it took around 0.3s

On a Postgres 12.14 DB with
1731 rows in FileInfo and 11M posts, it took around 0.27s
```

https://mattermost.atlassian.net/browse/MM-45956

server/channels/store/timerlayer/timerlayer.go

@@ -3182,10 +3182,10 @@ func (s *TimerLayerEmojiStore) Search(name string, prefixOnly bool, limit int) (
 	return result, err
 }
 
-func (s *TimerLayerFileInfoStore) AttachToPost(fileID string, postID string, creatorID string) error {
+func (s *TimerLayerFileInfoStore) AttachToPost(fileID string, postID string, channelID string, creatorID string) error {
 	start := time.Now()
 
-	err := s.FileInfoStore.AttachToPost(fileID, postID, creatorID)
+	err := s.FileInfoStore.AttachToPost(fileID, postID, channelID, creatorID)
 
 	elapsed := float64(time.Since(start)) / float64(time.Second)
 	if s.Root.Metrics != nil {