MM-38698: Add limit to the CLI compliance export command (#20997)

```release-note
A batchSize option has been added to the mattermost export
CLI command to limit the number of items exported. By default,
if it is not included, it exports all the posts.
```

https://mattermost.atlassian.net/browse/MM-38698


Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

einterfaces/message_export.go

@@ -11,5 +11,5 @@ import (
 
 type MessageExportInterface interface {
 	StartSynchronizeJob(ctx context.Context, exportFromTimestamp int64) (*model.Job, *model.AppError)
-	RunExport(format string, since int64) (int64, *model.AppError)
+	RunExport(format string, since int64, limit int) (int64, *model.AppError)
 }