MM-49374: Update job metadata for bulk export (#21983)

If a bulk export is triggered via a job, then
we update the metadata field of the job as it
progresses through the records.

https://mattermost.atlassian.net/browse/MM-49374

```release-note
Now you can monitor the progress of the bulk export job
via its metadata field. It is available at
`mmctl export job show <jobID>`.
```

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

cmd/mattermost/commands/export.go

@@ -240,7 +240,7 @@ func bulkExportCmdF(command *cobra.Command, args []string) error {
 	var opts model.BulkExportOpts
 	opts.IncludeAttachments = attachments
 	opts.CreateArchive = archive
-	if err := a.BulkExport(request.EmptyContext(a.Log()), fileWriter, filepath.Dir(outPath), opts); err != nil {
+	if err := a.BulkExport(request.EmptyContext(a.Log()), fileWriter, filepath.Dir(outPath), nil /* nil job since it's spawned from CLI */, opts); err != nil {
 		CommandPrintErrorln(err.Error())
 		return err
 	}