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/searchlayer/file_info_layer.go

@@ -112,8 +112,8 @@ func (s SearchFileInfoStore) SetContent(fileID, content string) error {
 	return err
 }
 
-func (s SearchFileInfoStore) AttachToPost(fileId, postId, creatorId string) error {
-	err := s.FileInfoStore.AttachToPost(fileId, postId, creatorId)
+func (s SearchFileInfoStore) AttachToPost(fileId, postId, channelId, creatorId string) error {
+	err := s.FileInfoStore.AttachToPost(fileId, postId, channelId, creatorId)
 	if err == nil {
 		nFileInfo, err2 := s.FileInfoStore.GetFromMaster(fileId)
 		if err2 == nil {