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/store.go

@@ -699,7 +699,7 @@ type FileInfoStore interface {
 	GetForUser(userID string) ([]*model.FileInfo, error)
 	GetWithOptions(page, perPage int, opt *model.GetFileInfosOptions) ([]*model.FileInfo, error)
 	InvalidateFileInfosForPostCache(postID string, deleted bool)
-	AttachToPost(fileID string, postID string, creatorID string) error
+	AttachToPost(fileID string, postID string, channelID, creatorID string) error
 	DeleteForPost(postID string) (string, error)
 	PermanentDelete(fileID string) error
 	PermanentDeleteBatch(endTime int64, limit int64) (int64, error)