MM-40818: Pass filestore to channels (#19677)

We move away from referencing server struct
but pass the filestore service to Channels
explicitly.

https://mattermost.atlassian.net/browse/MM-40818

```release-note
NONE
```

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

app/options.go

@@ -9,6 +9,7 @@ import (
 	"github.com/mattermost/mattermost-server/v6/config"
 	"github.com/mattermost/mattermost-server/v6/einterfaces"
 	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/shared/filestore"
 	"github.com/mattermost/mattermost-server/v6/shared/mlog"
 	"github.com/mattermost/mattermost-server/v6/store"
 )
@@ -65,6 +66,13 @@ func ConfigStore(configStore *config.Store) Option {
 	}
 }
 
+func SetFileStore(filestore filestore.FileBackend) Option {
+	return func(s *Server) error {
+		s.filestore = filestore
+		return nil
+	}
+}
+
 func RunEssentialJobs(s *Server) error {
 	s.runEssentialJobs = true