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/searchtest/helper.go

@@ -371,10 +371,11 @@ func (th *SearchTestHelper) createPost(userID, channelID, message, hashtags, pos
 	return th.Store.Post().Save(postModel)
 }
 
-func (th *SearchTestHelper) createFileInfoModel(creatorID, postID, name, content, extension, mimeType string, createAt, size int64) *model.FileInfo {
+func (th *SearchTestHelper) createFileInfoModel(creatorID, postID, channelID, name, content, extension, mimeType string, createAt, size int64) *model.FileInfo {
 	return &model.FileInfo{
 		CreatorId: creatorID,
 		PostId:    postID,
+		ChannelId: channelID,
 		CreateAt:  createAt,
 		UpdateAt:  createAt,
 		DeleteAt:  0,
@@ -387,12 +388,12 @@ func (th *SearchTestHelper) createFileInfoModel(creatorID, postID, name, content
 	}
 }
 
-func (th *SearchTestHelper) createFileInfo(creatorID, postID, name, content, extension, mimeType string, createAt, size int64) (*model.FileInfo, error) {
+func (th *SearchTestHelper) createFileInfo(creatorID, postID, channelID, name, content, extension, mimeType string, createAt, size int64) (*model.FileInfo, error) {
 	var creationTime int64 = 1000000
 	if createAt > 0 {
 		creationTime = createAt
 	}
-	fileInfoModel := th.createFileInfoModel(creatorID, postID, name, content, extension, mimeType, creationTime, size)
+	fileInfoModel := th.createFileInfoModel(creatorID, postID, channelID, name, content, extension, mimeType, creationTime, size)
 	return th.Store.FileInfo().Save(fileInfoModel)
 }